chore(Ring.Defs): Add docstrings for Semiring and Ring (#9941)

These were lost in the port.

Author: mattrobball

Mathlib/Algebra/Ring/Defs.lean

@@ -137,9 +137,13 @@ class NonAssocRing (α : Type*) extends NonUnitalNonAssocRing α, NonAssocSemiri
     AddCommGroupWithOne α
 #align non_assoc_ring NonAssocRing
 
+/-- A `Semiring` is a type with addition, multiplication, a `0` and a `1` where addition is
+commutative and associative, multiplication is associative and left and right distributive over
+addition, and `0` and `1` are additive and multiplicative identities. -/
 class Semiring (α : Type u) extends NonUnitalSemiring α, NonAssocSemiring α, MonoidWithZero α
 #align semiring Semiring
 
+/-- A `Ring` is a `Semiring` with negation making it an additive group. -/
 class Ring (R : Type u) extends Semiring R, AddCommGroup R, AddGroupWithOne R
 #align ring Ring