DefaultSettings.php: Update $wgPingback documentation
- Link to MediaWiki_Pingback_Privacy_Statement
- Link to aggregate pingback data on wmflabs
Change-Id: I71aaf97ea85f501bce391555b3f6ea984db16c50
(cherry picked from commit 03f9e687ce4b2efb1330e24677bb87e1b06e170d)