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/conversions/fill.rs

@@ -32,7 +32,7 @@ impl Command for Fill {
         "fill"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Fill and Align."
     }
 

crates/nu-command/src/conversions/into/binary.rs

@@ -43,7 +43,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to a binary primitive."
     }
 

crates/nu-command/src/conversions/into/bool.rs

@@ -29,7 +29,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to boolean."
     }
 

crates/nu-command/src/conversions/into/cell_path.rs

@@ -25,7 +25,7 @@ impl Command for IntoCellPath {
             .allow_variants_without_examples(true)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to a cell-path."
     }
 
@@ -33,7 +33,7 @@ impl Command for IntoCellPath {
         vec!["convert"]
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Converting a string directly into a cell path is intentionally not supported."
     }
 

crates/nu-command/src/conversions/into/command.rs

@@ -14,11 +14,11 @@ impl Command for Into {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Commands to convert data from one type to another."
     }
 
-    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/conversions/into/datetime.rs

@@ -145,7 +145,7 @@ impl Command for SubCommand {
         }
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert text or timestamp into a datetime."
     }
 

crates/nu-command/src/conversions/into/duration.rs

@@ -36,11 +36,11 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to duration."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "Max duration value is i64::MAX nanoseconds; max duration time unit is wk (weeks)."
     }
 

crates/nu-command/src/conversions/into/filesize.rs

@@ -51,7 +51,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to filesize."
     }
 

crates/nu-command/src/conversions/into/float.rs

@@ -32,7 +32,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert data into floating point number."
     }
 

crates/nu-command/src/conversions/into/glob.rs

@@ -39,7 +39,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to glob."
     }
 

crates/nu-command/src/conversions/into/int.rs

@@ -89,7 +89,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to integer."
     }
 

crates/nu-command/src/conversions/into/record.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to record."
     }
 

crates/nu-command/src/conversions/into/string.rs

@@ -60,7 +60,7 @@ impl Command for SubCommand {
             .category(Category::Conversions)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert value to string."
     }
 

crates/nu-command/src/conversions/into/value.rs

@@ -31,7 +31,7 @@ impl Command for IntoValue {
             .category(Category::Filters)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Infer nushell datatype for each cell."
     }