Improve typing docs

[alexfikl]

I tried to figure out that weird sphinx error (because I hit it in pytential as well):

<unknown>:1: WARNING: py:class reference target not found: ExpressionNode [ref.class]

From what I can tell, the issue was in

.. currentmodule:: pymbolic.typing
.. autoclass:: Expression

where Expression contained an ExpressionNode. Sphinx got confused because it was looking for ExpressionNode in the current module set to pymbolic.typing and failing. No idea why it would look there.. maybe because the annotation is a string and it didn't have any choice? The exact same thing works for ArithmeticExpression because ExpressionNode is in the same module (I think?).

Anyways, adding a fake reference fixes that sort of issue

.. class:: ExpressionNode

	See :class:`pymbolic.ExpressionNode`

Besides that, I moved some docs around because for some reason Sphinx wasn't showing them..

[inducer]

Thanks!