qdoc: Document a namespace in multiple modules

qdoc didn't handle this. This update fixes most of what was wrong,
but tuning the details of the namespace reference pages might follow.
We have namespace Qt as an example. Most of the elements in the Qt
namespace are in QtCore, but a few functions are declared in QtGui.
Before this update, qdoc used the hack of using #ifdef to remove the
declarations from qtextdocument.h in QtGui and .cpp and then added
them back into qtnamespace.h and .cpp in QtCore.

Now that hack is no longer necessary. The functions in the Qt namespace
that are declared in QtGui are documented there, but the documentation
is linked to from the namespace reference page, which remains in QtCore.
That is, only one \namespace command is used to document the Qt namespace,
and it appears in qnamespace.qdoc where it always did, but the documentation
for the Qt namespace functions declared in QtGui is now appears in
qtextdocument.cpp where it belongs.

This also allows qdoc to report when a namespace contains elements that are
public and documented, but the namespace itself is not documented, which was
not possible before this change. qdoc also reports if a namespace is documented
in more than one module. That is, for example, when \namespace Qt is used in
both QtCore and QtGui.

Note that this change will increase the number of qdoc warnings in
QtBase, but the new warnings are expacted.

Change-Id: If978a59209b7b2ae90713d3ae809ae03361df72f
Task-number: QTBUG-67267
Reviewed-by: Martin Smith <martin.smith@qt.io>