protocol: try to clarify wl_buffer doc

protocol: try to clarify wl_buffer doc

Fix few typos in wl_buffer description.

Mention backing storage in wl_buffer.destroy.

Try to clarify the wl_buffer.release semantics by not explaining what
*might* happen. It is important to not suggest, that if release does not
come before frame callback, it will not come before attaching a new
buffer to the surface. We want to allow the following scenario:

The compositor is able to texture from wl_buffers directly, but it also
keeps a copy of the surface contents. The copy is updated when the
compositor is idle, to avoid the performance hit on
wl_surface.attach/commit. When the copy completes some time later, the
server sends the release event. If the client has not yet allocated a
second buffer (e.g. it updates rarely), it can reuse the old buffer.

Reported-by: John Kåre Alsaker <john.kare.alsaker@gmail.com>
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>