In doing something else (Trac #14218) I tripped over the
definition of "naturally coherent" classes.  This patch

- Cocuments properly what that means

- Removes Typeable from the list, because now we know what
  it meams, Typeable clearly doesn't belong.

No regressions.

(Actually the term "naturally coherent" seems a bit off.
More like "invertible" or something.  But I left it.)