backport some fixes related to documentation building

backport some fixes related to documentation building

Origin: upstream, this is a backport of four commits:
 - https://code.qt.io/cgit/qt/qtbase.git/commit/?id=0014d81b1907621e
   doc: Revert to documenting the actual functions
 - https://code.qt.io/cgit/qt/qtbase.git/commit/?id=81d8319276f26d39
   doc: Fix all clang parse errors in QtBase during PCH build
 - https://code.qt.io/cgit/qt/qtbase.git/commit/?id=c306663990ba8681
   Fix qdoc errors in QProcess header
 - https://code.qt.io/cgit/qt/qtbase.git/commit/?id=98cb9275d064d8b9
   doc: clang reported two fake declarations to be the same
Last-Update: 2019-06-15

These changes help us to eliminate most of the warnings like this:
(qdoc) warning: No documentation for 'QRect::bottom() const'

There were 3603 such warnings without this patch, with it only 76
are left.

Gbp-Pq: Name docs_fixes.diff