Document TapHandler.tapped and [single|double]Tapped eventPoint argument

Amends b8fd580cb3453b3850c36765c4b2537538d2f4f8 to add documentation.
The eventPoint is important to get ephemeral state from the pointing
device: which button was released (thus triggering the tap), which
device it was, and where the release occurred.  Users may expect to use
the point property, but QQuickHandlerPoint::reset(QQuickEventPoint *)
resets every property of the point at the same time, so the architecture
currently does not allow for mixed state, i.e. having correct button
state but still holding leftover position information.  It may be
surprising for users, but the changes to the point property are an
atomic transaction that occurs before the signal.

Task-number: QTBUG-61749
Task-number: QTBUG-64847
Change-Id: I33e0e232084beba8e10d8b02fa3bf85f36293358
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

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