more cross-linking of equality

[shriram]

This is a very nice section:

https://www.pyret.org/docs/latest/equality.html#%28part._s~3aroughnum-equality%29

I think it would benefit from saying there are two kinds of comparisons possible, and linking to

https://www.pyret.org/docs/latest/equality.html#%28part._s~3abounded-equalities%29

I also wonder if the error message for roughnum equality couldn't link to the first link above? (Yes, it's controversial to have links in error messages… But it's such good prose, and a student who doesn't know what is going on is going to miss out on it.)