[jsx-handler-names] Document or remove undocumented special case for ref

[arild-haugstad]

With the current documentation, the resolution to #375 that enforces that callbacks for ref have prefix handle is somewhat surprising. It is currently not mentioned and not configurable; and I would argue that ref is not an event, hence its inclusion does not match the overall intent of the rule:
"Ensures that any component or prop methods used to handle events are correctly prefixed."

[yannickcr]

That's a bug, ref should be completely ignored.