firmware: rust: improve safety comments

Improve the wording of safety comments to be more explicit about what
exactly is guaranteed to be valid.

Suggested-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://lore.kernel.org/r/20240619132029.59296-1-dakr@redhat.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>