Add the note about data size to QByteArray::operator=(const char*)

Add the note about the way the str size is determined when using
QByteArray::operator=(const char*).

Pick-to: 6.7 6.6 6.5
Change-Id: I39b2d0fc2967832622fbf0c11b3ff6c7ff99b8f2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/corelib/text/qbytearray.cpp

@@ -1349,6 +1349,9 @@ QByteArray &QByteArray::operator=(const QByteArray & other) noexcept
     \overload
 
     Assigns \a str to this byte array.
+
+    \a str is assumed to point to a null-terminated string, and its length is
+    determined dynamically.
 */
 
 QByteArray &QByteArray::operator=(const char *str)