Document a11y event types in Input.js

[zepumph]

@pixelzoom brought the lack of documentation to light today in the a11y-dev channel. Items to do:

1. Add documentation at the top of the file, like around the Event Types section.
2. Add better documentation to A11Y_EVENT_TYPES
3. Ask @pixelzoom to review

[zepumph]

Note from meetings today with @jessegreenberg and @emily-phet. Regarding #939, I wonder if we shouldn't put a fair bit of information regarding how certain a11y related keyboard events behave (a) given a certain tag name, and (b) depending on if a screen reader is active.

[pixelzoom]

The more documentation, the better. Also useful to identify things that are a11y standards vs PhET standards.

[zepumph]

@pixelzoom please review.

[pixelzoom]

Looks great, thanks!