Add back QWheelEvent position() and globalPosition() docs

Change 7d298072

 removed the docs for
the obsolete pos() and globalPos() accessors, but the text should have
been reused to document their replacements.

Change-Id: If4d64e0f07666a99d9a0a4f0de9fca42d3acf0f8
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>