Document more event dispatch quirks

[annevk]

Everything is terrible:

• disabled form controls are special: Mouse events & disabled form controls html#2368 (Chrome is attempting to remove this, but is alone thus far and it's not enabled by default; it's also a little unclear at what layer this needs to be specified, but some browsers implement it as part of dispatch)
• submit/reset events on form elements need to be handled in a special way: Figure out if submit and reset events need to be handled in special way during dispatch html#3599 and Synthetic submit event should probably trigger form submission html#4438

[miketaylr]

Another related bug affecting Yahoo Mail: https://bugzilla.mozilla.org/show_bug.cgi?id=1220048

(possibly related too? https://bugzilla.mozilla.org/show_bug.cgi?id=654072)