Explicitly clarify naming policy for React event handler methods

The [ordering section](https://github.com/airbnb/javascript/tree/master/react#ordering) implies that event handlers should be named `onFoo` rather than `handleFoo` per FB examples. Is this an explicit rule? If so, it would be nice to have it listed in either the "naming" or "methods" sections.

Start with the linked React ordering section and compare its event-handler guidance with the naming and methods sections. Confirm with maintainers whether onFoo or handleFoo is the intended policy, then document the explicit rule in the agreed section and verify the style guide wording is consistent.

Written by the indexing model from the issue text.