Doc: Removed the \relates instances

QDoc supports \relates in the context of a function that
relates to an existing class or a namespace, but not in the context
of a class. We can use \sa to list the related classes or namespaces
for a class instead.

This change ensures that QDoc generates documentation for QMessageLogger
and QMessageLogContext classes.

Task-number: QTBUG-28468

Change-Id: I2242ab730fe5e3acf54b6fa65774e751d2daa7a4
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/corelib/global/qlogging.cpp

@@ -83,7 +83,6 @@ static bool isFatal(QtMsgType msgType)
 /*!
     \class QMessageLogContext
     \inmodule QtCore
-    \relates <QtGlobal>
     \brief The QMessageLogContext class provides additional information about a log message.
     \since 5.0
 
@@ -96,7 +95,6 @@ static bool isFatal(QtMsgType msgType)
 /*!
     \class QMessageLogger
     \inmodule QtCore
-    \relates <QtGlobal>
     \brief The QMessageLogger class generates log messages.
     \since 5.0