Rewrote the subsection on coercion rules (and made it a proper
subsection, with a label).  The new section is much less precise,
because precise rules would be too hard to give (== I don't know what
they are any more :-).  OTOH, the new section gives much more
up-to-date information.

Also noted that __coerce__ may return NotImplemented, with the same
meaning as None.

I beg Fred forgiveness: my use of \code{} is probably naive.  Please
fix this and other markup nits.  An index entry would be nice.

This could be a 2.2 bugfix candidate, if we bother about old docs
(Fred?)