Moved the section about generating absolute URLs down

This is so we have the controller/PHP version and template version in the same spot. See #3573

I also made a few other tweaks, including using generateUrl (like we did earlier) to generate
the absolute URL, not going through the router service directly.
Fixing typo thanks to @xabbuh
minor #3693 Tweak to Absolute URL generation (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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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!

Commits
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007a54f Fixing typo thanks to @xabbuh
873706c Moved the section about generating absolute URLs down