bug#48768: Introducing ability to append to docstring

[ludvig-faddeev]

A useful capability would allow one to append to a defun docstring using a 
defvar.

User could write a brief description for the docstring, and later append more 
details
from a defvar.

I like my functions to be succinct without an overly long docstring. This makes
the implementation code readily displayed on the visible window, without having
to scroll through an excessively long documentation string.