Edit rm help messages

[webbedspace]

Description

Splitoff from #6983. This tweaks rm's help messages for grammar and readability.

Also, this adds a test showing a slightly more complex rm use-case.

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 --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all tests pass

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.

[rgwood]

Nice, thanks. Think a couple rm_always_trash mentions in the examples should be replaced with always_trash, otherwise LGTM.