Documenting it as a callable is a pretty strong hint that it shouldn't be subclassed, but I agree my wording above is wrong. Next attempt (leaving out the mention of leading underscores, since that is covered elsewhere):

=================
Class Names
-----------

Class names should normally use the CapWords convention.

The naming convention for functions may be used instead in cases where the interface is documented and used primarily as a callable.

Note that there is a separate convention for builtin names: most builtin names are single words (or two words run together), with the CapWords convention used only for exception names and builtin constants.
=================

I initially had the following appended to the second paragraph: "... , and instances expose no public mutable attributes or instance methods (but may provide read-only data attributes, alternative constructors and appropriate special methods)".

The longer phrasing was designed to explicitly cover itertools.chain.from_iterable, contextlib.redirect_stdio.target and contextlib.suppress.exceptions, but I don't think that level of detail is necessary.