Summary:
Add examples for `Data.Bool`, and rework the existing documentation of
the `bool` function slightly: the `a`,`b` in its explanation were changed
to `x`,`y` to avoid ambiguity with the type variable 'a'.

The examples have been tested, and two trailing spaces were removed.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D360