Fixes #1624.

This fixes #1521

 - added terse syntax to modules.rst
 - added description of special variables to template module

Reverting as this as reported allows some variables to clobber each other.
We'll work on a different fix.

This reverts commit c5469f63.

- The old documentation stated not to quote the variable in the function, which is obviously wrong. When using lists or integers, we have to quote otherwise the startswith() method will trip over its non-existence.
- It was unclear that the only_if statement is processed for each item when doing with_items, so I added an example making it crystal clear how this can be used in your advantage (or why a non-existing list variable can break your logic if you expected the only_if to be processed once before running the task)

I had trouble finding this information, so though it was worth
adding to docs... but am not sure its easy to find for someone
specifically to find.  At least I think this inclusion matches the
tone of document, but maybe this info should go into a FAQ
instead/as-well?

  add ./hacking/