QByteArray: Add a note regarding overlapping pointers to qstr(n)cpy

Stated e.g. in http://en.cppreference.com/w/c/string/byte/strcpy

Change-Id: I42fd5a5fa6a63b67a7105aa56e93e3d3f2193cf7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qbytearray.cpp

@@ -261,6 +261,8 @@ char *qstrdup(const char *src)
     This function assumes that \a dst is large enough to hold the
     contents of \a src.
 
+    \note If \a dst and \a src overlap, the behavior is undefined.
+
     \sa qstrncpy()
 */
 
@@ -292,6 +294,8 @@ char *qstrcpy(char *dst, const char *src)
     This function assumes that \a dst is at least \a len characters
     long.
 
+    \note If \a dst and \a src overlap, the behavior is undefined.
+
     \note When compiling with Visual C++ compiler version 14.00
     (Visual C++ 2005) or later, internally the function strncpy_s
     will be used.