doc(set_theory/cardinal/basic): tweak documentation (#15480)
doc(set_theory/cardinal/basic): tweak documentation (#15480)

This PR does the following:
- remove the mention of `cardinal.min`, as it was removed in #13410.
- elaborate on `cardinal.sum`.
- add mention of `cardinal.prod`.
- separate the documentation of definitions with that of instances.




leanprover-community/mathlib,leanprover-community/mathlib