Doc: Fix incorrect \fn signatures

Several \fn signatures were either incorrect or missing qualifiers
required to make QDoc match the documentation with the declarations.

These were not caught in the CI when testing documentation builds due
to a bug in QDoc.

Manual conflict resolution for 6.8:
- dropped the qhash.cpp changes (they were in API new in 6.9)
- dropped the qstring.cpp changes (they were in API changed for 6.9)

Task-number: QTBUG-134250
Change-Id: I7a4af11d9bc3687597b348cc2d4ee15212e0281f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit b6a01d7d22583263a16242af604e74bbe694d766)
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
(cherry picked from commit f08721bb3962125f4207a5dccdb4415990161084)
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit b3c0c09981a54f81feec83e092ed41dd104c3ab8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qsocketnotifier.cpp

@@ -173,7 +173,7 @@ QSocketNotifier::~QSocketNotifier()
 
 
 /*!
-    \fn void QSocketNotifier::activated(int socket)
+    \fn void QSocketNotifier::activated(int socket, QPrivateSignal)
     \deprecated To avoid unintended truncation of the descriptor, use
     the QSocketDescriptor overload of this function. If you need
     compatibility with versions older than 5.15 you need to change

src/gui/math3d/qmatrix4x4.cpp

@@ -729,8 +729,7 @@ QMatrix4x4 operator/(const QMatrix4x4& matrix, float divisor)
 }
 
 /*!
-    \fn bool qFuzzyCompare(const QMatrix4x4& m1, const QMatrix4x4& m2)
-    \relates QMatrix4x4
+    \fn bool QMatrix4x4::qFuzzyCompare(const QMatrix4x4& m1, const QMatrix4x4& m2)
 
     Returns \c true if \a m1 and \a m2 are equal, allowing for a small
     fuzziness factor for floating-point comparisons; false otherwise.