per 4054 linked to next/last from while/until; moved return etc later

[arkiuat]

The problem

The docs for loop-control keywords next and last etc were listed at the very end of Language/control.rakudoc, separated from the main discussion of loop-control syntax by the sections on fail, return, return-rw. Issue https://github.com/Raku/doc/issues/4054 requested we add a link to these even though they're in the same document.

Solution provided

Added a link, and also moved the sections fail, return, return-rw to the end of the document so that they didn't come in the middle of the discussion of loop-control syntax.