pyinotify: configure checks and documentation

After 74d519e3b91845a17ae095eb7d58dd9e3d1303e8 Ganeti depends on
pyinotify. Updating the documentation accordingly and checking for its
presence at configure time.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>