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/formats/to/command.rs

@@ -8,7 +8,7 @@ impl Command for To {
         "to"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Translate structured data to a format."
     }
 
@@ -18,7 +18,7 @@ impl Command for To {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    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/formats/to/csv.rs

@@ -65,7 +65,7 @@ impl Command for ToCsv {
         ]
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert table into .csv text ."
     }
 

crates/nu-command/src/formats/to/json.rs

@@ -28,7 +28,7 @@ impl Command for ToJson {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts table data into JSON text."
     }
 

crates/nu-command/src/formats/to/md.rs

@@ -27,7 +27,7 @@ impl Command for ToMd {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert table into simple Markdown."
     }
 

crates/nu-command/src/formats/to/msgpack.rs

@@ -25,11 +25,11 @@ impl Command for ToMsgpack {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert Nu values into MessagePack."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"
 Not all values are representable as MessagePack.
 

crates/nu-command/src/formats/to/msgpackz.rs

@@ -34,11 +34,11 @@ impl Command for ToMsgpackz {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert Nu values into brotli-compressed MessagePack."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "This is the format used by the plugin registry file ($nu.plugin-path)."
     }
 

crates/nu-command/src/formats/to/nuon.rs

@@ -31,7 +31,7 @@ impl Command for ToNuon {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts table data into Nuon (Nushell Object Notation) text."
     }
 

crates/nu-command/src/formats/to/text.rs

@@ -24,7 +24,7 @@ impl Command for ToText {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Converts data into simple text."
     }
 

crates/nu-command/src/formats/to/toml.rs

@@ -16,7 +16,7 @@ impl Command for ToToml {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert record into .toml text."
     }
 

crates/nu-command/src/formats/to/tsv.rs

@@ -34,7 +34,7 @@ impl Command for ToTsv {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert table into .tsv text."
     }
 

crates/nu-command/src/formats/to/xml.rs

@@ -38,7 +38,7 @@ impl Command for ToXml {
             .category(Category::Formats)
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"Every XML entry is represented via a record with tag, attribute and content fields.
 To represent different types of entries different values must be written to this fields:
 1. Tag entry: `{tag: <tag name> attributes: {<attr name>: "<string value>" ...} content: [<entries>]}`
@@ -89,7 +89,7 @@ Additionally any field which is: empty record, empty list or null, can be omitte
         ]
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert special record structure into .xml text."
     }
 

crates/nu-command/src/formats/to/yaml.rs

@@ -15,7 +15,7 @@ impl Command for ToYaml {
             .category(Category::Formats)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Convert table into .yaml/.yml text."
     }