Add an example ITemplateHelpers extension
Improve the ITemplateHelpers docstrings

Just make the docstrings a bit clearer and more explicit, and reference
the new example_itemplatehelpers extension.
example_itemplatehelpers use a function not a method

It's not a good idea for template helper functions to be class methods,
because self is not thread-safe.
Add a couple of details to ITemplateHelpers docs