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/math/abs.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the absolute value of a number."
     }
 

crates/nu-command/src/math/avg.rs

@@ -30,7 +30,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the average of a list of numbers."
     }
 

crates/nu-command/src/math/ceil.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the ceil of a number (smallest integer greater than or equal to that number)."
     }
 

crates/nu-command/src/math/floor.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the floor of a number (largest integer less than or equal to that number)."
     }
 

crates/nu-command/src/math/log.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the logarithm for an arbitrary base."
     }
 

crates/nu-command/src/math/math_.rs

@@ -14,11 +14,11 @@ impl Command for MathCommand {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Use mathematical functions as aggregate functions on a list of numbers or tables."
     }
 
-    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/math/max.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the maximum of a list of values, or of columns in a table."
     }
 

crates/nu-command/src/math/median.rs

@@ -24,7 +24,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Computes the median of a list of numbers."
     }
 

crates/nu-command/src/math/min.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Finds the minimum within a list of values or tables."
     }
 

crates/nu-command/src/math/mode.rs

@@ -54,7 +54,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the most frequent element(s) from a list of numbers or tables."
     }
 

crates/nu-command/src/math/product.rs

@@ -24,7 +24,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the product of a list of numbers or the products of each column of a table."
     }
 

crates/nu-command/src/math/round.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the input number rounded to the specified precision."
     }
 

crates/nu-command/src/math/sqrt.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the square root of the input number."
     }
 

crates/nu-command/src/math/stddev.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the standard deviation of a list of numbers, or of each column in a table."
     }
 

crates/nu-command/src/math/sum.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the sum of a list of numbers or of each column in a table."
     }
 

crates/nu-command/src/math/variance.rs

@@ -25,7 +25,7 @@ impl Command for SubCommand {
             .category(Category::Math)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Returns the variance of a list of numbers or of each column in a table."
     }