drm/doc: Clarify what ioctls can be used on render nodes

The documentation for render nodes indicates that only "PRIME-related"
ioctls are valid on render nodes, but the documentation does not clarify
what that means.  If the reader is not familiar with PRIME, they may
beleive this to be only the ioctls with "PRIME" in the name and not other
ioctls such as set of syncobj ioctls.  Clarify the situation for the
reader by referencing where the reader will find a current list of valid
ioctls.

Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1646667156-16366-1-git-send-email-quic_jhugo@quicinc.com