Single string literal in body is return value only, not doc string

A function or macro body consisting of a single string literal now only
uses it as a return value.  Previously, it had the dual uses as return
value and doc string, which was never what the programmer wanted and
had some inconvenient consequences (bug#69387).

This change applies to `lambda`, `defun`, `defsubst` and `defmacro`
forms; most other defining forms already worked in the sensible way.

* lisp/emacs-lisp/bytecomp.el (byte-compile-lambda):
Don't use a lone string literal as doc string.
* test/lisp/emacs-lisp/bytecomp-resources/warn-wide-docstring-defun.el
(foo): Update docstring warning test.
* doc/lispref/functions.texi (Function Documentation): Update.
* etc/NEWS: Announce.

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