nouveau/gsp: document some aspects of GSP-RM

Document a few aspects of communication with GSP-RM. These comments are
derived from notes made during early development of GSP-RM support in
Nouveau, but were not included in the initial patch set.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231122202840.2565153-1-ttabi@nvidia.com