Add quick overview for ado notation in Applicative

Author: JordanMartinez

src/Control/Applicative.purs

@@ -29,6 +29,38 @@ import Data.Unit (Unit, unit)
 -- | - Composition: `pure (<<<) <*> f <*> g <*> h = f <*> (g <*> h)`
 -- | - Homomorphism: `(pure f) <*> (pure x) = pure (f x)`
 -- | - Interchange: `u <*> (pure y) = (pure (_ $ y)) <*> u`
+-- |
+-- | ## Ado Notation
+-- |
+-- | When using a type that has an instance for `Applicative`, one can use
+-- | "ado notation." In short, this code...
+-- | ```
+-- | foo =
+-- |   functionThatTakes4Args <$> boxedA
+-- |                          <*> boxedB
+-- |                          <*> boxedUnit
+-- |                          <*> boxedC
+-- |   where
+-- |   functionThatTakes4Args a b _ c =
+-- |     let x = a * a + b
+-- |     in x + c
+-- | ```
+-- |
+-- | ... can be converted into into "ado notation:"
+-- | ```
+-- | foo = ado
+-- |   a <- boxedA
+-- |   b <- boxedB
+-- |   boxedUnit
+-- |   c <- boxedC
+-- |   let x = a * a + b
+-- |   in x + c
+-- | ```
+-- |
+-- | Note: if one wants to use "ado notation" but redefine what the in-scope
+-- | definitions are for `map`, `apply`, and `pure` in a given context, one can
+-- | use "qualified ado notation." This is an intermediate/advanced language
+-- | feature not explained here.
 class Apply f <= Applicative f where
   pure :: forall a. a -> f a