QRegularExpression: fix optimizePattern, document the issue

QRegularExpression: fix optimizePattern, document the issue

The studyData pointer is atomically set by the pointer assignment,
but another processor running a different thread might see the
new studyData value but not the memory it points to.

Therefore, the current studyData is returned from optimizePattern
and used by that thread.

Docs were added to optimizePattern to explain what's going on.

Change-Id: I4502c336077bb98a1751011aa93ffd4f585ed101
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>