Doc: make a note about use of QStringLiteral with non-ASCII chars

Both ICC and MSVC have bugs concatenating strings and that's despite the
standard giving an example ([lex.string] table 9) that says that
  u"a" "b"
is the same as
  u"ab"

We can't change to preprocessor concatenation (u ## STR) because that
fails when QStringLiteral is used with a macro instead of an actual
string literal.

Task-number: QTBUG-65479
Change-Id: I39332e0a867442d58082fffd1504a0a868c291ef
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstring.cpp

@@ -10802,7 +10802,7 @@ QString QString::toHtmlEscaped() const
   This cost can be avoided by using QStringLiteral instead:
 
   \code
-  if (node.hasAttribute(QStringLiteral("http-contents-length"))) //...
+  if (node.hasAttribute(QStringLiteral(u"http-contents-length"))) //...
   \endcode
 
   In this case, QString's internal data will be generated at compile time; no
@@ -10822,6 +10822,10 @@ QString QString::toHtmlEscaped() const
   if (attribute.name() == QLatin1String("http-contents-length")) //...
   \endcode
 
+  \note Some compilers have bugs encoding strings containing characters outside
+  the US-ASCII character set. Make sure you prefix your string with \c{u} in
+  those cases. It is optional otherwise.
+
   \sa QByteArrayLiteral
 */