drm/doc: Polish kerneldoc for encoders

- Move missing bits into struct drm_encoder docs.
- Explain that encoders are 95% internal and only 5% uapi, and that in
  general the uapi part is broken.
- Remove verbose comments for functions not exposed to drivers.

v2: Review from Archit:
- Appease checkpatch in the moved code.
- Make it clearer that bridges are not exposed to userspace.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-2-daniel.vetter@ffwll.ch