DomStrReplace: Add regex backreference example

I submitted a MR with a new code example, based on something I wrote/used myself earlier today.

(It's not exactly like str_replace, simply because the code examples in dom_str_replace are structured differently -- with less extensive explanations -- and I didn't restructure what's in dom_str_replace. So, that's why that is.)

(See "Issue summary" > "Proposed resolution" for an alternative idea.)

Extra examples in documentation are usually a good thing. I reviewed this one, and it is not an exception.

Thanks for making it easier for the next person trying to make sense of the DOM plugins!