python: Remove 'del g' from the docs, replace with 'g.close ()'.

'del g' is a trap for the unwary.  If the handle has any other
references, it does nothing (in fact, it can be actively dangerous if
the user was expecting the appliance to go away).  In non-CPython it
can be delayed arbitrarily long.

Using 'g.close()' on the other hand is always safe.

python/examples/create_disk.py

@@ -58,4 +58,4 @@ g.upload ("/etc/resolv.conf", "/foo/resolv.conf")
 # Because 'autosync' was set (above) we can just close the handle
 # and the disk contents will be synchronized.  You can also do
 # this manually by calling g.umount_all and g.sync.
-del g
+g.close ()