Explain what the QStringView levels mean

Insofar the information was just in the commit message
(00a8be85d1); add that explanation
to the code itself.

Change-Id: Ice1919c803a31918e8228091b3192728cbbc32f4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstringview.h

@@ -40,6 +40,18 @@
 #ifndef QSTRINGVIEW_H
 #define QSTRINGVIEW_H
 
+/*
+    This macro enables three "levels" of QStringView support:
+
+    1. offer QStringView, overload some functions taking QString with
+    QStringView
+
+    2. like 1, but remove all overloads of functions taking QStringRef,
+    leaving only the function taking QStringView.  Do this only where
+    QStringRef overloads tradionally existed.
+
+    3. like 2, but replace functions taking QString, too.
+*/
 #ifndef QT_STRINGVIEW_LEVEL
 #  define QT_STRINGVIEW_LEVEL 1
 #endif