Update typos in "Set Methods"

 * Corrected typo in the Result column for `isdisjoint` by changing an 'is' into an 'if'
 * Corrected error in theResult column for `issuperset`. All the elements of the set passed in the argument are supposed to be
   in the set on which the method was called. This was most likely a typo due to copy and pasting :)
 * Made format for all the set methods that return a Boolean value consistent.

Resolves mattharrison#9

Author: shadyan

python.rst

@@ -732,9 +732,9 @@ Sets are useful because they provide *set operations*, such as union
   ``s.discard(item)``                                               Remove ``item`` from s (mutates ``s``). No error on missing ``item``
   ``s.intersection(s2)``                                            Return set with elements from both sets
   ``s.intersection_update(s2)``                                     Update ``s`` with members of ``s2`` (mutates ``s``)
-  ``s.isdisjoint(s2)``                                              ``True`` is there is no intersection
-  ``s.issubset(s2)``                                                All elements of ``s`` in ``s2``
-  ``s.issuperset(s2)``                                              All elements of ``s2`` in ``s2``
+  ``s.isdisjoint(s2)``                                              ``True`` if there is no intersection of these two sets
+  ``s.issubset(s2)``                                                ``True`` if all elements of ``s`` are in ``s2``
+  ``s.issuperset(s2)``                                              ``True`` if all elements of ``s2`` are in ``s``
   ``s.pop()``                                                       Remove arbitrary item from s (mutates ``s``). ``KeyError`` on missing ``item``
   ``s.remove(item)``                                                Remove ``item`` from s (mutates ``s``). ``KeyError`` on missing ``item``
   ``s.symmetric_difference(s2)``                                    Return set with elements only in one of the sets