bug#69387: 30.0.50; A string shouldn't be both a docstring and a return value

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

Package: Emacs
Version: 30.0.50


Currently, ELisp defines

    (lambda (blabla) "Help!")

as a function that returns "Help!" *and* whose docstring is "Help!".
As seen in commit eeb89a5cb292bffe40ba7d0b0cf81f82f8452bf8, it can be
a source of annoyance as well.

I cannot remember finding source code which makes use of this "feature".
My impression is that our docs document this behavior simply because
that's how it happened to work rather than how it should work.
This is documented in the texinfo under "Function Documentation" where
it says:

    [...
    effects, it has no effect if it is not the last form in the body.  Thus,
    in practice, there is no confusion between the first form of the body
    and the documentation string; if the only body form is a string then it
    serves both as the return value and as the documentation.

I think we should change that to say that

    if the only body form is a string then it serves as the return value
    and not as the documentation.

This will/would require a few changes to `macroexp.el` and
`bytecomp.el`, but it should be minor.  It shouldn't introduce any
significant breakage either because the only effect will be to make it
so some functions won't have a docstring any more, but most (all?) of
those function never expected to have a docstring in the first place.


        Stefan