Fix up some QMatrix4x4 docs

The deprecation note for one of the operator* is wrong: Using
mapVector() is never the right choice given the deprecated operator*
itself is implemented by calling map(). It could be that in some cases
the results are identical so one can get away with migrating to
mapVector(), but this is incorrect in other cases and causes
regressions all over the place. Thus we should only recommend using
map() instead, never mapVector(). As shown by recent experiences, the
current docs are insufficient/wrong and caused a number of regressions
in the Qt code base as well.

Also enhance some related docs. It could be that the wording is not
always academically correct, however having some additional notes in
there help make it obvious to the reader (or at least raise some
alarms) that there are important differences between map and
mapVector.

Also improve the note for the other deprecated operator*. There is
only one way to convert to a vec4, given the existing implementation
of the operator, and that is by using 1 for w. Make this clear to the
reader.

Pick-to: 6.3 6.2
Change-Id: I1d8dbca44fdc103ab62d49bfc1d4ce37a9bc130b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

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