bpo-27470: Improve doc for commandline -3 option

Mention that it warns about features that are significantly changed in Python 3
and can’t be detected using static code analysis.

Link to Porting Python 2 Code to Python 3 doc

Doc/using/cmdline.rst

@@ -417,10 +417,12 @@ Miscellaneous options
 
    Warn about Python 3.x possible incompatibilities by emitting a
    :exc:`DeprecationWarning` for features that are removed or significantly
-   changed in Python 3.
+   changed in Python 3 and can't be detected using static code analysis.
 
    .. versionadded:: 2.6
 
+   See :doc:`/howto/pyporting` for more details.
+
 Options you shouldn't use
 ~~~~~~~~~~~~~~~~~~~~~~~~~