drm/doc: updates for new framebuffer lifetime rules

Now that framebuffer are reference-counted for all use-sites, update
the documentation accordingly to stress the new rules for
initialization and teardown.

Also add a short paragraph about the implications for drivers of the
new locking rules.

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>