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!

Thanks for your contributions!

Automatically closed - issue fixed for 2 weeks with no activity.