Add extra_usage messages for subcommand-only commands (#7594)

# Description

The message reads "You must use one of the following subcommands. Using
this command as-is will only produce this help message." and is added to
commands like `into`, `bytes`, `str`, etc.

# User-Facing Changes

See above.

# 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-command/src/path/path_.rs

@@ -22,7 +22,9 @@ impl Command for PathCommand {
     }
 
     fn extra_usage(&self) -> &str {
-        r#"There are three ways to represent a path:
+        r#"You must use one of the following subcommands. Using this command as-is will only produce this help message.
+        
+There are three ways to represent a path:
 
 * As a path literal, e.g., '/home/viking/spam.txt'
 * As a structured path: a table with 'parent', 'stem', and 'extension' (and