Document how UIAUTOMATION.POSTEVENT works.
Also update UIAUTOMATION.POSTKEYTOWINDOW to prove that :event can be a list.
UIAUTOMATION.POSTEVENT is going to be used by the MESSAGEBOX tests to get the position of a modal dialog box.
Authored by: david_costanzo 2020-12-11
Parent: [r5517]
Child: [r5519]
