Improve QString doc when using non-spaced numbered place marker

Currently when a developer uses a string like
QString("%1%3%2").arg(x).arg(y).arg(z) he can be bitten by the
sequential replacement done by QString. Adding an example with a little
explanation should help future Qt user avoid generating buggy strings.

Task-number: QTBUG-44044
Change-Id: I81e20af8d9fb2a07e12ec61dcd5bb4544d863777
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>

src/corelib/doc/snippets/qstring/main.cpp

@@ -261,6 +261,19 @@ void Widget::argFunction()
     str.arg("%1f").arg("Hello");    // returns "Hellof %2"
     //! [13]
 
+    //! [97]
+    str = "%1%3%2";
+    str.arg("Hello").arg(20).arg(50); // returns "Hello500"
+
+    str = "%1%2%3";
+    str.arg("Hello").arg(50).arg(20); // returns "Hello5020"
+    //! [97]
+
+    //! [98]
+    str = "%1%2%3";
+    str.arg("Hello", QString::number(20), QString::number(50)); // returns "Hello5020"
+    //! [98]
+
     //! [14]
     str = QString("Decimal 63 is %1 in hexadecimal")
             .arg(63, 0, 16);

src/corelib/tools/qstring.cpp

@@ -7206,6 +7206,25 @@ QString QString::arg(const QString &a, int fieldWidth, QChar fillChar) const
   difference if \a a1 contains e.g. \c{%1}:
 
   \snippet qstring/main.cpp 13
+
+  A similar problem occurs when the numbered place markers are not
+  white space separated:
+
+  \snippet qstring/main.cpp 12
+  \snippet qstring/main.cpp 97
+
+  Let's look at the substitutions:
+  \list
+  \li First, \c Hello replaces \c {%1} so the string becomes \c {"Hello%3%2"}.
+  \li Then, \c 20 replaces \c {%2} so the string becomes \c {"Hello%320"}.
+  \li Since the maximum numbered place marker value is 99, \c 50 replaces \c {%32}.
+  \endlist
+  Thus the string finally becomes \c {"Hello500"}.
+
+  In such cases, the following yields the expected results:
+
+  \snippet qstring/main.cpp 12
+  \snippet qstring/main.cpp 98
 */
 
 /*!