> +   ``hg clone http://hg.python.org/cpython``;
> I use http://hg.python.org/cpython#default to avoid cloning the 2.x 
> branches, that’s an optimization that can save some time and space.

I think in the default case it's easier to get'em all.

> +2. run ``./configure --with-pydebug && make -j2``
> Is make -j2 harmless on single-CPU systems?  Doesn’t our make default 
> to -j0 anyway, to use all CPUs?

As far as I know, -j2 is harmless on single-CPU systems, but I don't think we use all the CPUs by default with a plain `make`.

> +3. :doc:`run the tests <runtests>` with ``./python -m test``;
> Is that cross-platform?  Mac OS X has python.exe for example.

These instructions are a bit UNIX/Linux-centric, because that's what most developers seems to use.  On Mac OS it would indeed be ./python.exe, but it shouldn't take long for the developers to figure that out or click on the link and see how it works for their OS.

If you think it's worth to expand a bit, I guess we could still do it.