qdoc: Show additional comments that are attached to \obsolete or \deprecated members

Functions/enums that are marked "\obsolete" or "\deprecated" are sometimes annotated with additional comments that show the relevant replacements. Examples:

\obsolete Use QMetaType::Type instead
\enum QVariant::Type

\property QTextEdit::tabStopWidth
\deprecated in Qt 5.10. Use tabStopDistance instead.

(see http://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/kernel/qvariant.cpp#n1374, http://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/widgets/qtextedit.cpp#n1960 )

However, the hints don't appear in the final documentation (see http://doc.qt.io/qt-5/qvariant-obsolete.html#Type-enum, http://doc.qt.io/qt-5/qtextedit-obsolete.html )

These comments would be helpful to developers who want to future-proof their code.