Fix typos, add an xref

[kamahen]

No description provided.

[JanWielemaker]

Thanks. Removed hint at PL_get_compound_name_arity(t,NULL,NULL). It is not true (it is the same as PL_is_compound(); PL_is_functor() checks that a terms is compound and has a particular functor) and I find it mostly confusing. PL_get_size_ex() reference to PL_get_intptr() is a bit dubious. Probably there should be a PL_get_size(), although I don't think there are any sensible cases for using it as you almost always want the type and domain check. The C standard does not guarantee size_t and uintptr_t to be the same (although they are on almost all platforms).