Subject: 26.3; Function usage in docstring not documented

From: Allen Li <[email-masked]>
Date: Sun, 10 Nov 2019 01:48:24 -0800

It is not documented in the Elisp info manual that a function docstring
ending with:

\(fn SOME ARGS)

will have a custom usage displayed by `describe-function'.

It seems useful to document this.

In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.10)
 of 2019-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
System Description:	Arch Linux