Split messages for recent change legend and letter tooltips

This makes it possible to use formatting in the legend ones
(e.g. links to help pages) and to use different wording
(the one for tooltips is sometimes too verbose for the legend).

Documented the format of $wgRecentChangesFlags better.

The default text in not modified in this change.

Bug: 58715
Bug: 60235
Change-Id: Ie9872c8fb3e43620877300ee8536bb5ddee5ab46