Add some more clarification on why event_extra can't be too large
Add some more clarification on why event_extra can't be too large

Change-Id: I3b366e06bb1980d834cad09c24dde929981ea366

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo