Clarify how register works (#8583)

# Description

This PR changes some help text to try and clarify how `register` works
when using from `nu -c`.


# 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

> **Note**
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> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
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> toolkit check pr
> ```

# 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-cmd-lang/src/core_commands/register.rs

@@ -63,7 +63,7 @@ impl Command for Register {
                 result: None,
             },
             Example {
-                description: "Register `nu_plugin_query` plugin from `nu -c`(plugin will be available in that nu session only)",
+                description: "Register `nu_plugin_query` plugin from `nu -c` (writes/updates $nu.plugin-path)",
                 example: r#"let plugin = ((which nu).path.0 | path dirname | path join 'nu_plugin_query'); nu -c $'register ($plugin); version'"#,
                 result: None,
             },