Show match index in place of match count

This is the implementation of https://trello.com/c/0GXqt5GF/1089-s-find-next-indicate-current-match-index.
@JeffryBooher I would love to see this cleaned up, but I think it falls outside the scope of this PR.  There are probably 100+ instances of CM pos & range structures written in our docs, so we should do one big cleanup and fix all of them (rather than only fixing this one line of new code as a one-off).  That way, we can also pick a nicer global location to the typedef declaration, double-check that the generated apify docs still look good, etc...