feat: doc-strings for grind algebra classes (#8990)

This PR adds missing doc-strings for grind's internal algebra
typeclasses, for inclusion in the reference manual.

src/Init/Grind/Module/Basic.lean

@@ -11,7 +11,11 @@ import Init.Grind.ToInt
 
 namespace Lean.Grind
 
+/--
+A type where addition is right-cancellative, i.e. `a + c = b + c` implies `a = b`.
+-/
 class AddRightCancel (M : Type u) [Add M] where
+  /-- Addition is right-cancellative. -/
   add_right_cancel : ∀ a b c : M, a + c = b + c → a = b
 
 /--
@@ -204,8 +208,14 @@ end IntModule
 /--
 We say a module has no natural number zero divisors if
 `k ≠ 0` and `k * a = k * b` implies `a = b` (here `k` is a natural number and `a` and `b` are element of the module).
+
+For a module over the integersm this is equivalent to
+`k ≠ 0` and `k * a = 0` implies `a = 0`.
+(See the alternative constructor `NoNatZeroDivisors.mk'`,
+and the theorem `eq_zero_of_mul_eq_zero`.)
 -/
 class NoNatZeroDivisors (α : Type u) [HMul Nat α α] where
+  /-- If `k * a ≠ k * b` then `k ≠ 0` or `a ≠ b`.-/
   no_nat_zero_divisors : ∀ (k : Nat) (a b : α), k ≠ 0 → k * a = k * b → a = b
 
 export NoNatZeroDivisors (no_nat_zero_divisors)

src/Init/Grind/Ring/Field.lean

@@ -10,10 +10,17 @@ import Init.Grind.Ring.Basic
 
 namespace Lean.Grind
 
+/--
+A field is a commutative ring with inverses for all non-zero elements.
+-/
 class Field (α : Type u) extends CommRing α, Inv α, Div α where
+  /-- Division is multiplication by the inverse. -/
   div_eq_mul_inv : ∀ a b : α, a / b = a * b⁻¹
+  /-- Zero is not equal to one: fields are non trivial.-/
   zero_ne_one : (0 : α) ≠ 1
+  /-- The inverse of zero is zero. This is a "junk value" convention. -/
   inv_zero : (0 : α)⁻¹ = 0
+  /-- The inverse of a non-zero element is a right inverse. -/
   mul_inv_cancel : ∀ {a : α}, a ≠ 0 → a * a⁻¹ = 1
 
 attribute [instance 100] Field.toInv Field.toDiv