(PUP-2794) Update Callable Type specification

PUP-2794 introduces the ability to specify a Callable with Union
type (ignored type). The specification only shows the result of this
internal use of Union, the Union type is not documented as it is an
implementation detail.

This updates the specification as the type algebra of Callable
has changed in accordance with PUP-2794.
s/that/who/ because we are talking about programmers, and programmers are mostly people and we refer to people as *who*, and things as *that* — that's not a hard rule or anything, it just helps the parsing of complex sentences… Just like types ;)