qstrncpy(): document that we don't implement strncpy() fill-all-[dst,len)

The Windows version didn't, ever since we started using strncpy_s(),
and we might change the non-Windows version, too.

Pick-to: 6.5 6.4 6.2 5.15
Change-Id: Iee1c09674d296be806d4be42b1891940120b4a01
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/text/qbytearray.cpp

@@ -123,6 +123,10 @@ char *qstrcpy(char *dst, const char *src)
 
     \note If \a dst and \a src overlap, the behavior is undefined.
 
+    \note Unlike strncpy(), this function does \e not write '\\0' to all \a
+    len bytes of \a dst, but stops after the terminating '\\0'. In this sense,
+    it's similar to C11's strncpy_s().
+
     \sa qstrcpy()
 */