Documentation: explain the difference between __bitwise and __bitwise__

Simply added explanation from Al Viro in the following mail:
http://lkml.indiana.edu/hypermail/linux/kernel/0802.2/3164.html

Cc: Al Viro <viro@ftp.linux.org.uk>
Cc:  Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>