Issue #7537: Add is_disjoint() method and an alias isdisjoint()

Add a method for finding if two sets are disjoint.
On Fri, Jun 06, 2014 at 07:56:28AM -0700, Randy wrote:
>    Couldn't this just be isdisjoint = is_disjoint? That would automatically
>    give them both the same docstring without having to crossreference. And
>    would save four lines.

Will be the isdisjoint method mentioned in the sphinx docs
for this case?