Documentation: Add a warning to "socket options"

Documentation: Add a warning to "socket options"

 Add a warning to the "socket options" section of the
 smb.conf man page that changing socket options can be
 dangerous to performance.  Hopefully this will cut down on
 users reporting poor performance after changing socket
 options.

Signed-off-by: Scott Lovenberg <scott.lovenberg@gmail.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>