rust/drm: Fix Registration::{new,new_foreign_owned}() docs

rust/drm: Fix Registration::{new,new_foreign_owned}() docs

Looks like we've actually had a malformed rustdoc reference in the rustdocs
for Registration::new_foreign_owned() for a while that, when fixed, still
couldn't resolve properly because it refers to a private item.

This is probably leftover from when Registration::new() was public, so drop
the documentation from that function and fixup the documentation for
Registration::new_foreign_owned().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Fixes: 0600032c54b7 ("rust: drm: add DRM driver registration")
Cc: <stable@vger.kernel.org> # v6.16+
Link: https://patch.msgid.link/20260122221037.3462081-1-lyude@redhat.com