Improve template.dd documentation

This PR contains both:
- a reflection of the already fixed compiler [issue 3467](https://d.puremagic.com/issues/show_bug.cgi?id=3467)
- a reflection of the not yet merged compiler enhancement https://github.com/D-Programming-Language/dmd/pull/3156 for  [issue 2025](https://d.puremagic.com/issues/show_bug.cgi?id=2025)

So, don't merge this until the compiler enhancement will be accepted.