Add a explanation for function templates that is missing

Thanks for the expalanation given by H. S. Teoh at http://forum.dlang.org/post/mailman.1476.1415061701.9932.digitalmars-d-learn@puremagic.com
This is wrong, `template` is not a storage class. This is a very unclear way to describe function templates. A much better approach is to introduce a dedicated section somewhere else on this page to explain the basics of function templates.