drm/bridge: Improve overview documentation

Clean up the drm_bridge overview documentation, and expand the
operations documentation to provide more details on API usage.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-7-laurent.pinchart@ideasonboard.com