document + improve ancient optimization in ParseXS

From commit 8990e30710 "perl 5.0 alpha 6". The DESTROY XSUB with T_PTROBJ
as INPUT arg was refusing to obey my custom typemap for T_PTROBJ, but all
other XSUBs followed the custom T_PTROBJ override. Extend this
optimization to other rarely used typemap types and document this behavior.