drm/doc: document the type plane property

Add a new entry for "type" in the section for standard plane properties.

v3: improve paragraph about mixing legacy IOCTLs with explicit usage,
note that a driver may support cursors without cursor planes (Daniel)

v4: fixing rebase gone wrong

v5:
- Fix typo (Daniel)
- Mention CAP_ATOMIC instead of CAP_UNIVERSAL_PLANES when referring to
  atomic test-only commits (Daniel)
- Add newlines at end of sections (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115110626.12233-2-contact@emersion.fr