Doc: Update description of QKeyEvent class

Update the description on how to use the QKeyEvent pointer
passed to key event handlers, and remove an outdated note about
multimedia key events.

Change-Id: I67a3f0054e28b84d5a0e367c02a329f4670221c7
Task-number: QTBUG-35155
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>