bug#26272: default-directory: Remark that it must be a directory name

[Tino Calancha]

The docstring of `file-name-as-directory' suggests that is
important that `default-directory' be a directory name, i.e.,
for instance in GNU/Linux ends with '/'.  The docstring of
`default-directory' doesn't mention it.

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>From 1579eeb2388d65c4fe65fb37e4e5abe373de69e3 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Mon, 27 Mar 2017 19:04:29 +0900
Subject: [PATCH] default-directory: Remark that it must be a directory name

* src/buffer.c (default-directory): Update docstring.
---
 src/buffer.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/buffer.c b/src/buffer.c
index c1c53dd220..f0c334f08b 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5754,7 +5754,10 @@ visual lines rather than logical lines.  See the 
documentation of
   DEFVAR_PER_BUFFER ("default-directory", &BVAR (current_buffer, directory),
                     Qstringp,
                     doc: /* Name of default directory of current buffer.
-To interactively change the default directory, use command `cd'.  */);
+It must be a directory name, as the output of `file-name-as-directory'.
+To interactively change the default directory, use command `cd'.
+In a Elisp program is better to do
+(setq default-directory (file-name-as-directory DIR)) */);
 
   DEFVAR_PER_BUFFER ("auto-fill-function", &BVAR (current_buffer, 
auto_fill_function),
                     Qnil,
-- 
2.11.0

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In GNU Emacs 25.2.4 (x86_64-pc-linux-gnu, GTK+ Version 3.22.9)
 of 2017-03-27
Repository revision: fb18bff91f01a3051253319e766ca276e5b756bd