doc: add historical comments about definition of LE.

Game/MyNat/LE.lean

@@ -4,13 +4,26 @@ namespace MyNat
 
 def le (a b : ℕ) :=  ∃ (c : ℕ), b = a + c
 
--- Another choice is to define it recursively:
+/-
--- (kb) note: I didn't choose this option because tests showed
--- that mathematicians found it a lot more confusing than
--- the existence definition.
 
--- | le 0 _
+Another choice would have been to define `MyNat.le` recursively, like `Nat.le` is
--- | le (succ a) (succ b) = le ab
+defined in core:
+
+```
+protected inductive Nat.le (n : Nat) : Nat → Prop
+  /-- Less-equal is reflexive: `n ≤ n` -/
+  | refl     : Nat.le n n
+  /-- If `n ≤ m`, then `n ≤ m + 1`. -/
+  | step {m} : Nat.le n m → Nat.le n (succ m)
+```
+
+Yet another option would be the definition I tried in Exercise 9 of the 2017 prototype version
+of the game (see https://xenaproject.wordpress.com/2017/10/31/building-the-non-negative-integers-from-scratch/).
+
+I ultimately didn't choose these options because tests showed
+that mathematicians found building the basic API a lot more confusing than
+with the existence definition.
+-/
 
 -- notation
 instance : LE MyNat := ⟨MyNat.le⟩