[ADT] Fix FoldingSet documentation typos

* "If found then M with be non-NULL" should be "will be non-NULL".
* The documentation examples (1) and (2) declare and use a variable
  `MyNode *M`, but examples (3) and (4) switch midway to using a
  variable named `N`. Unify the examples to all use `M`.
* The examples demonstrate the use of member functions of
  `FoldingSet`, but (3) and (4) invoke these as if they were free
  functions. Modify them to call member functions on the `MyFoldingSet`
  object constructed in the code above example (1).