Doc: Improve documentation for QObject::findChild

There is a difference between findChild() and findChild("") that was
not explained in the documentation. So, add a sentence to clarify it.

The object's search when there are several direct ancestors was
documented as "undefined". Change it to "first child in children()".

Task-number: QTBUG-103986
Pick-to: 6.6 6.5
Change-Id: I25cb01b87d3b24af498ad494b3f353840ee78807
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/kernel/qobject.cpp

@@ -1986,14 +1986,15 @@ void QObject::killTimer(int id)
 
     Returns the child of this object that can be cast into type T and
     that is called \a name, or \nullptr if there is no such object.
-    Omitting the \a name argument causes all object names to be matched.
+    A null \a name argument causes all objects to be matched. An empty,
+    non-null \a name matches only objects whose \l objectName is empty.
     The search is performed recursively, unless \a options specifies the
     option FindDirectChildrenOnly.
 
-    If there is more than one child matching the search, the most
-    direct ancestor is returned. If there are several direct
-    ancestors, it is undefined which one will be returned. In that
-    case, findChildren() should be used.
+    If there is more than one child matching the search, the most-direct
+    ancestor is returned. If there are several most-direct ancestors, the
+    first child in children() will be returned. In that case, it's better
+    to use findChildren() to get the complete list of all children.
 
     This example returns a child \c{QPushButton} of \c{parentWidget}
     named \c{"button1"}, even if the button isn't a direct child of