Tweak to Absolute URL generation

| Q | A |
|----------------|------|
| Doc fix | yes |
| New docs | no |
| Applies to | 2.3 |
| Fixed tickets | #3573  |

This updates how we describe URL generation. After #3573, it occurs to me that if you're looking here for a reference, you'll look for the first spot where absolute URLs are discussed and try to use it. So why not put the controller/PHP version and template version of doing this in the same spot.

Cheers!