QObject: stronger warning about isSignalConnected and threads

Document explicitly that it is not allowed to call isSignalConnected
from (dis)connectNotify overrides, and add the respective warning from
the disconnectNotify documentation also to the connectNotify
documentation (with some light editing).

Fixes: QTBUG-106025
Change-Id: I41e8a9d3e6ce697cb2943d55a7c853eeec9c1dbe
Reviewed-by: Lars Knoll <lars@knoll.priv.no>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit e75c1a00e31723f1c9deb8427725fa0a58fae2a8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qobject.cpp

@@ -2653,13 +2653,15 @@ int QObject::receivers(const char *signal) const
 
     \snippet code/src_corelib_kernel_qobject.cpp 49
 
-    As the code snippet above illustrates, you can use this function
+    As the code snippet above illustrates, you can use this function to avoid
-    to avoid emitting a signal that nobody listens to.
+    expensive initialization or emitting a signal that nobody listens to.
+    However, in a multithreaded application, connections might change after
+    this function returns and before the signal gets emitted.
 
     \warning This function violates the object-oriented principle of
-    modularity. However, it might be useful when you need to perform
+    modularity. In particular, this function must not be called from an
-    expensive initialization only if something is connected to a
+    override of connectNotify() or disconnectNotify(), as those might get
-    signal.
+    called from any thread.
 */
 bool QObject::isSignalConnected(const QMetaMethod &signal) const
 {
@@ -3333,8 +3335,13 @@ bool QObject::disconnect(const QObject *sender, const QMetaMethod &signal,
     signal.
 
     \warning This function is called from the thread which performs the
-    connection, which may be a different thread from the thread in
+    connection, which may be a different thread from the thread in which
-    which this object lives.
+    this object lives. This function may also be called with a QObject internal
+    mutex locked. It is therefore not allowed to re-enter any QObject
+    functions, including isSignalConnected(), from your reimplementation. If
+    you lock a mutex in your reimplementation, make sure that you don't call
+    QObject functions with that mutex held in other places or it will result in
+    a deadlock.
 
     \sa connect(), disconnectNotify()
 */
@@ -3363,12 +3370,12 @@ void QObject::connectNotify(const QMetaMethod &signal)
     expensive resources.
 
     \warning This function is called from the thread which performs the
-    disconnection, which may be a different thread from the thread in
+    disconnection, which may be a different thread from the thread in which
-    which this object lives. This function may also be called with a QObject
+    this object lives. This function may also be called with a QObject internal
-    internal mutex locked. It is therefore not allowed to re-enter any
+    mutex locked. It is therefore not allowed to re-enter any QObject
-    of any QObject functions from your reimplementation and if you lock
+    functions, including isSignalConnected(), from your reimplementation. If
-    a mutex in your reimplementation, make sure that you don't call QObject
+    you lock a mutex in your reimplementation, make sure that you don't call
-    functions with that mutex held in other places or it will result in
+    QObject functions with that mutex held in other places or it will result in
     a deadlock.
 
     \sa disconnect(), connectNotify()