Documentation/gpu: Clarify format of driver-specific fidnfo keys

This change reflects de facto usage by amdgpu, as exemplified by commit
d6530c33 ("drm/amdgpu: expose more memory stats in fdinfo").

Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250130172851.941597-2-adrian.larumbe@collabora.com