Doc: update Project Options section in qmake Manual

Explain which mkspecs have the configuration option
precompile_header set.

Task-number: QTBUG-25683

Change-Id: I9b11eafad16b65523cbb90897fe88829add58bea
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/ad4bda8b754a5fb4f7c6d5c4c3b941579df83c41)