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 a good example, but should be explained more clearly in its own section.

Also, all code examples must follow [Phobos coding style](http://dlang.org/dstyle.html).