staging/android: add Doc for SW_SYNC ioctl interface

staging/android: add Doc for SW_SYNC ioctl interface

This interface is hidden from kernel headers and it is intended for use
only for testing. So testers would have to add the ioctl information
internally. This is to prevent misuse of this feature.

v2: take in Eric suggestions for the Documentation

v3: really take in Eric suggestions

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>