Feeds for recent changes now provide URLs for the change (i.e., the diff), not just...

Feeds for recent changes now provide URLs for the change (i.e., the diff), not just the page.  This matches behavior of history feeds, which appears more correct.  Incidentally, this code needs centralization, probably, say into a constructor method of Revision or Article.  Logic is unnecessarily duplicated at present.  (Also tweaked some docs, used __METHOD__ instead of $fname.)