rust: drm: Improve safety comment when using `Pin::into_inner_unchecked`

rust: drm: Improve safety comment when using `Pin::into_inner_unchecked`

The safety requirements for `Pin::into_inner_unchecked` state that the
returned pointer must be treated as pinned until it is dropped. Such a
guarantee is provided by the `ARef` type. This patch improves the safety
comment to better reflect this.

Signed-off-by: Ewan Chorynski <ewan.chorynski@ik.me>
Link: https://patch.msgid.link/20251228-drm-gem-safety-comment-v2-1-99bb861c3371@ik.me
Signed-off-by: Danilo Krummrich <dakr@kernel.org>