Recursive compare machinery: The code that intended to exempt tuples

Recursive compare machinery:  The code that intended to exempt tuples
was broken because new-in-2.3 code added a tp_as_mapping slot to tuples.
Repaired that.

Added basic docs to check_recursion().

The code that intended to exempt tuples and strings was also broken here,
and in 2.2:  these should use PyXYZ_CheckExact(), not PyXYZ_Check() -- we
can't know whether subclass instances are immutable.  This part (and this
part alone) is a bugfix candidate.