RE: [External] : Supplying DOC string in a `defun' using `defvar'

[Drew Adams]

> That feature is kind of hidden, as the only place where it is
> mentioned in the manual is at (elisp) Generic Functions
>
>      If DOCUMENTATION is present (it should always be),
>      it specifies the documentation string for the generic 
>      function, in the form ‘(:documentation DOCSTRING)’.

https://lists.gnu.org/archive/html/help-gnu-emacs/2021-06/msg00027.html