QMake: Document PRECOMPILED_DIR

The variable was not documented. In projects that build both debug
and release, OBJECTS_DIR does not affect the location of the
precompiled header, because qmake falls back to the debug or release
directory before it can use OBJECTS_DIR as the default.

Describe PRECOMPILED_DIR and its default, and point out how to
place the precompiled header in builds with both configurations.

Fixes: QTBUG-54441
Pick-to: 6.8 6.12
Change-Id: I25f1c943237615ea0d8c676c9dc8f069d24eb063
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>