[media] doc-rst: document LIRC_GET_*_TIMEOUT ioctls

Improve the documentation for those ioctls, adding them to
a separate file, in order to look like the rest of the
book, and to later allow to generate a man page for those
ioctls.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>