protocol: fix clarification of input region on drags and pointers

The previous clarification did not follow the current implementation in
Weston, where when a surface stops being a cursor or an icon, it becomes
a plain unmapped surface again.

Rewrite the related paragraphs, and fix some typos while at it.

For start drag, make it explicit of which surface argument we are
talking about.

v2:

Make the input region undefined when the use ends. Most likely no-one
will re-use these surfaces for anything else than the same use case, so
leave some slack for the implementations to avoid useless work on
resetting the regions.

Reported-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>