Updates to bring template docs in line with Puppet 3 deprecations

In https://projects.puppetlabs.com/issues/19058 we deprecated the
bareword method_missing syntax for using variables in ERB templates.
This commit brings the template guide in line with the current
usage guidelines and deprecation cycle.

There's a copy-edit or two for clarity in here, too.
Merge pull request #176 from ahpook/master

Update templating guide for deprecations