Update porting docs
Update porting docs

Some improvements to the QML engine in Qt 5 lead to behavior changes.
These should be explained in the porting guide.

Task-number: QTBUG-28411
Change-Id: Ifc4016cf7b33ba113dbe0cbabd2f71a4ff1a8685
Reviewed-by: hjk <6fac1c92f5b9c7890a04d7f8a9d8912d16355eac@ovi.com>
Reviewed-by: Bea Lam <3b8b412f85e38e2fbf2a7d737c89d787c51aca40@jollamobile.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <e4034124b095075783ce739a2d1c68c0d66fa1fd@digia.com>

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