Groups and rings tweaks

[PatrickMassot]

From Jireh on Zulip about the groups and rings chapter:

1. Early on when discussing morphisms, the proofs used f.map_mul or f.map_inv as opposed to map_mul f and map_inv f. Of course this is minor, but at the same time, it doesn't seem like these kinds of generic hom-class lemmas were discussed in the previous chapter (7.2: Hierarchies - Morphisms), so maybe the point to do it is here?
2. Upon introducing the polynomial notation R[X], I think that special emphasis should be placed on the fact that this is only syntax, the X has no meaning. It's not like R[Y] gives you polynomials in a different indeterminate. The same goes for Polynomial.X which gets abbreviated to X in the examples. On a cursory read, I can imagine a newcomer (especially students) being confused about the fact that Y doesn't work if you replace X everywhere.