Incorporate documentation suggestions from feedback on comp.lang.python.

Incorporate documentation suggestions from feedback on comp.lang.python.

* Positive wording for the description of why < and > and = can all
  be False.

* Move to a three column table format that puts long method names
  side-by-side with their operator equivalents

* Mention that KeyError can be raised by Set.pop() and Set.remove().

* Minor tweaks to the examples.

Will backport as soon as Fred rebuilds the docs so I can confirm
the tables formatted properly