Add level 3 header for overview of Monoid/Semigroup newtypes

Author: JordanMartinez

src/Data/Monoid.purs

@@ -29,6 +29,8 @@ import Type.Data.RowList (RLProxy(..))
 -- | `<>` is used to combine individual results, and `mempty` gives the result
 -- | of folding an empty collection of elements.
 -- |
+-- | ### Newtypes for Monoid
+-- |
 -- | Some types (e.g. `Int`, `Boolean`) can implement multiple law-abiding
 -- | instances for `Monoid`. For example, `<>` could be `+` and `mempty` could
 -- | be `0`. Likewise, `<>` could be `*` and `mempty` could be `1`.

src/Data/Semigroup.purs

@@ -21,6 +21,8 @@ import Type.Data.RowList (RLProxy(..))
 -- | concatenation. Another example is `List a`, with `(<>)` defined as
 -- | list concatenation.
 -- |
+-- | ### Newtypes for Semigroup
+-- |
 -- | There are two other ways to implement an instance for this type class
 -- | regardless of which type is used. These instances can be used by
 -- | wrapping the values in one of the two newtypes below: