More documentation regarding zero_type/void/intp is needed

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1656

Reference: http://pike.roxen.com/documentation/tutorial/tutorial_4.html#4.1.1

Reported by Petter Isaksson, petter.isaksson@home.se

How and when to use zero_type is far from obvious, I just stumbled upon the following:

Pike v7.0 release 304 running Hilfe v2.0 (Incremental Pike Frontend)
> int testfun(string s, void|int i)
>> {
>> if (zero_type(i)) write("i is zero_type");
>> if (intp(i)) write("\n i is integer\n");
>> }
> testfun("a");
i is zero_type
 i is integer
Result: 0
> testfun("a",17);

i is integer Result: 0

For me, when writing a function like foo(int|void arg), the obvious choice is to pick intp() to check if arg is supplied or not, and the result is certainly not the expected.

Or perhaps this is a bug in (the documentation of) intp()? :) (See url)