Change the usage misnomer to "description" (#13598)

# Description
    
The meaning of the word usage is specific to describing how a command
function is *used* and not a synonym for general description. Usage can
be used to describe the SYNOPSIS or EXAMPLES sections of a man page
where the permitted argument combinations are shown or example *uses*
are given.
Let's not confuse people and call it what it is a description.

Our `help` command already creates its own *Usage* section based on the
available arguments and doesn't refer to the description with usage.

# User-Facing Changes

`help commands` and `scope commands` will now use `description` or
`extra_description`
`usage`-> `description`
`extra_usage` -> `extra_description`

Breaking change in the plugin protocol:

In the signature record communicated with the engine.
`usage`-> `description`
`extra_usage` -> `extra_description`

The same rename also takes place for the methods on
`SimplePluginCommand` and `PluginCommand`

# Tests + Formatting
- Updated plugin protocol specific changes
# After Submitting
- [ ] update plugin protocol doc

crates/nu-command/src/system/complete.rs

@@ -15,11 +15,11 @@ impl Command for Complete {
             .input_output_types(vec![(Type::Any, Type::record())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Capture the outputs and exit code from an external piped in command in a nushell table."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"In order to capture stdout, stderr, and exit_code, externally piped in commands need to be wrapped with `do`"#
     }
 

crates/nu-command/src/system/exec.rs

@@ -16,11 +16,11 @@ impl Command for Exec {
             .category(Category::System)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Execute a command, replacing or exiting the current process, depending on platform."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"On Unix-based systems, the current process is replaced with the command.
 On Windows based systems, Nushell will wait for the command to finish and then exit with the command's exit code."#
     }

crates/nu-command/src/system/nu_check.rs

@@ -25,7 +25,7 @@ impl Command for NuCheck {
             .category(Category::Strings)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Validate and parse input content."
     }
 

crates/nu-command/src/system/ps.rs

@@ -26,7 +26,7 @@ impl Command for Ps {
             .category(Category::System)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about system processes."
     }
 

crates/nu-command/src/system/registry_query.rs

@@ -42,11 +42,11 @@ impl Command for RegistryQuery {
             .category(Category::System)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Query the Windows registry."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Currently supported only on Windows systems."
     }
 

crates/nu-command/src/system/run_external.rs

@@ -23,7 +23,7 @@ impl Command for External {
         "run-external"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Runs external command."
     }
 

crates/nu-command/src/system/sys/cpu.rs

@@ -17,7 +17,7 @@ impl Command for SysCpu {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system CPUs."
     }
 

crates/nu-command/src/system/sys/disks.rs

@@ -17,7 +17,7 @@ impl Command for SysDisks {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system disks."
     }
 

crates/nu-command/src/system/sys/host.rs

@@ -18,7 +18,7 @@ impl Command for SysHost {
             .input_output_types(vec![(Type::Nothing, Type::record())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system host."
     }
 

crates/nu-command/src/system/sys/mem.rs

@@ -16,7 +16,7 @@ impl Command for SysMem {
             .input_output_types(vec![(Type::Nothing, Type::record())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system memory."
     }
 

crates/nu-command/src/system/sys/net.rs

@@ -17,7 +17,7 @@ impl Command for SysNet {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system network interfaces."
     }
 

crates/nu-command/src/system/sys/sys_.rs

@@ -15,11 +15,11 @@ impl Command for Sys {
             .input_output_types(vec![(Type::Nothing, Type::record())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the system."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "You must use one of the following subcommands. Using this command as-is will only produce this help message."
     }
 

crates/nu-command/src/system/sys/temp.rs

@@ -16,11 +16,11 @@ impl Command for SysTemp {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View the temperatures of system components."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Some system components do not support temperature readings, so this command may return an empty list if no components support temperature."
     }
 

crates/nu-command/src/system/sys/users.rs

@@ -16,7 +16,7 @@ impl Command for SysUsers {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View information about the users on the system."
     }
 

crates/nu-command/src/system/uname.rs

@@ -15,7 +15,7 @@ impl Command for UName {
             .category(Category::System)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Print certain system information using uutils/coreutils uname."
     }
 

crates/nu-command/src/system/which_.rs

@@ -20,7 +20,7 @@ impl Command for Which {
             .category(Category::System)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Finds a program file, alias or custom command."
     }