Add comments for nu syntax shape (#7349)

# Description

FIx the typo of `List` and also add more comments to other variants


# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-protocol/src/syntax_shape.rs

@@ -55,7 +55,7 @@ pub enum SyntaxShape {
     /// A table is allowed, eg `[[first, second]; [1, 2]]`
     Table,
 
-    /// A table is allowed, eg `[first second]`
+    /// A list is allowed, eg `[first second]`
     List(Box<SyntaxShape>),
 
     /// A filesize value is allowed, eg `10kb`
@@ -67,7 +67,7 @@ pub enum SyntaxShape {
     /// A datetime value, eg `2022-02-02` or `2019-10-12T07:20:50.52+00:00`
     DateTime,
 
-    /// An operator
+    /// An operator, eg `+`
     Operator,
 
     /// A math expression which expands shorthand forms on the lefthand side, eg `foo > 1`
@@ -77,7 +77,7 @@ pub enum SyntaxShape {
     /// A general math expression, eg `1 + 2`
     MathExpression,
 
-    /// A variable name
+    /// A variable name, eg `$foo`
     Variable,
 
     /// A variable with optional type, `x` or `x: int`
@@ -89,10 +89,10 @@ pub enum SyntaxShape {
     /// A general expression, eg `1 + 2` or `foo --bar`
     Expression,
 
-    /// A boolean value
+    /// A boolean value, eg `true` or `false`
     Boolean,
 
-    /// A record value
+    /// A record value, eg `{x: 1, y: 2}`
     Record,
 
     /// An error value