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
Using personal pronouns (arguably except "we") in documentation is just unprofessional in general. Hopefully we don't need an explicit rule to enforce that.