Docs: non-schema attributes

[dan-fritchman]

https://github.com/dan-fritchman/Hdl21#spice-class-simulation

Says:

Class-based Sim definitions retain all class members which are SimAttrs and drop all others. Non-SimAttr-valued fields can nonetheless be handy for defining intermediate values upon which the ultimate SimAttrs depend, such as the a_path field in the example above.

• This is also true of Module and Bundle, and it's not clearly stated anywhere.
• Story changes if #182 produces a policy change
• To noone in particular: RTFM