[issue4087] Document the effects of NotImplemented on == and !=

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

Looks good.  Thanks, Raymond.

Minor typos:  "numberic" -> "numeric"
"the expression ``x in y`` equivalent to" -> 
"the expression ``x in y`` is equivalent to"

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<http://bugs.python.org/issue4087>
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