[backport#17015] qa: Explain QT_QPA_PLATFORM for gui tests

Summary:
fa06bb607da2d3e35723661156d873c8eac1fa50 qa: Do not force overwrite of QT_QPA_PLATFORM on windows for gui tests (MarcoFalke)
faccf5f9c899c40d4da5792629d0714249a4616b doc: Explain QT_QPA_PLATFORM for gui tests (MarcoFalke)

Pull request description:

Closes #17013

Depends on D7748

Backport of Core PR17015

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7749