Provide docstrings for horizon tables DeleteAction

While it's true that the `horizon.tables.actions.DeleteAction`
code is self explanatory (and hence documentation on its own),
the motivation behind this was to replace """Doc missing.""" with
something more descriptive that will show up at:-
http://docs.openstack.org/developer/horizon/ref/tables.html#horizon.tables.DeleteAction

It would also be good to call `help(DeleteAction)` to get a decent
description of the class.

Change-Id: Ib1198711589ac704278db53d4d53fd655146aaa9
Merge "Provide docstrings for horizon tables DeleteAction"