[docs] QLatin1String: add some guidance as to when to use it

Change-Id: I391be8bda3a5cb4873b89b437d2b76b1cd88261f
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>

src/corelib/tools/qstring.cpp

@@ -8222,6 +8222,13 @@ QString &QString::setRawData(const QChar *unicode, int size)
 
     \snippet code/src_corelib_tools_qstring.cpp 6
 
+    \note If the function you're calling with a QLatin1String
+    argument isn't actually overloaded to take QLatin1String, the
+    implicit conversion to QString will trigger a memory allocation,
+    which is usually what you want to avoid by using QLatin1String
+    in the first place. In those cases, using QStringLiteral may be
+    the better option.
+
     \sa QString, QLatin1Char, {QStringLiteral()}{QStringLiteral}
 */