I think 'Builtin-in Types' should probably be 'Built-in Classes' as there is no longer any difference between types and classes. Note that after the unification, we added 'issubclass', not 'issubtype'.

type should at least be listed under types/classes. The one-parameter introspection use returns an existing instance of type rather than a new instance. In this, it is similar to bool, which returns an existing rather than new bool instance, and which is also used for introspection. I think both should be listed as classes, which they are, and then the intro to Introspection should mention that bool(ob) and type(ob) both return information about ob by returning existing instances of bool and type respectively.

I thought about whether we should merely add a categorized index or also re-arrange the entire page. It it the possibly of adding section paragraphs, including cross-references like the above, that pushes me in the latter direction. The math section intro could mention that functions of collections of numbers are in the Iterables section section.