drm/doc: document how userspace should find out CRTC index

In this patch we add a section to document what userspace should do to
find out the CRTC index. This is important as they may be many places in
the documentation that need this, so it's better to just point to this
section and avoid repetition.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609230039.73307-2-leandro.ribeiro@collabora.com