bug#73431: Add `setf` support for `stream.el` in ELPA

[Michael Heerdegen via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> > Did you also want to delete the documentation string of `seq-mapn`,
> > which contains an example?
>
> I can't find the patch right now to make an informed judgment, but the
> docstring of the stream method of `seq-mapn` should be specific to the
> stream method.
> Please look at the result in `C-h f seq-mapn` to see if your
> docstring makes sense.

Dunno how this could be beautified to harmonize better with the C-h f
output BUT we definitely do want to keep the Fibonacci number
construction example because it is very educative.  And it fits well
here in this docstring, so, modulo cosmetic improvements, I would let it
be.

Michael.