Update docs

Author: gusty

docsrc/content/abstraction-foldable.fsx

@@ -58,6 +58,8 @@ From .Net/F#
  -  ``seq<'T>``
  -  ``list<'T>``
  -  ``'T []``
+ -  ``Set<'T>``
+ -  ``HashSet<'T>``
  -  ``option<'T>`` 
  -  ``voption<'T>`` 
  -  ``ResizeArray<'T>`` 

docsrc/content/abstraction-functor.fsx

@@ -104,6 +104,7 @@ Restricted:
  -  ``string``
  -  ``StringBuilder``
  -  ``Set<'T>``
+ -  ``HashSet<'T>``
  -  ``IEnumerator<'T>``
  [Suggest another](https://github.com/fsprojects/FSharpPlus/issues/new) concrete implementation
 

docsrc/content/abstraction-monoid.fsx

@@ -57,6 +57,7 @@ From .Net/F#
  -  ``StringBuilder``
  -  ``unit``
  -  ``Set<'T>``
+ -  ``HashSet<'T>``
  -  ``Map<'T,'Monoid>``
  -  ``TimeSpan`` 
  -  ``Tuple<'Monoid1* ... *'MonoidN>``

docsrc/content/abstraction-semigroup.fsx

@@ -42,6 +42,7 @@ From .Net/F#
  -  ``StringBuilder``
  -  ``unit``
  -  ``Set<'T>``
+ -  ``HashSet<'T>``
  -  ``Map<'T, 'U>``
  -  ``TimeSpan`` 
  -  ``Tuple<*>``