Clean-up bisect docs.

Clean-up bisect docs.
* Document the post conditions for bisect_left() and bisect_right().
* Fix the grades example to use more obvious breakpoints
  and to use a list comprehension instead of map() which returns
  an iterator in Py3.x.
* Improve and expand the examples of searching sorted lists.
* Issue 4356 -- move up reference to the SortedCollection recipe.