Backport Guido's changes from revision 1.89:

Backport Guido's changes from revision 1.89:

Addressed SF bug 421973 (finally).

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.

FLD: My modifications to Guido's markup are included from revision 1.90.