staging/android: Add some more kerneldoc to ashmem.c

I am sorry if I have interpreted anything incorrectly here. This is my
second day really attempting to understand the Ashmem system.

I can not finish documenting this class at this stage - There is still
more that I have to learn. For now, however, it will have to do.

Signed-off-by: Cruz Julian Bishop <cruzjbishop@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>