Improve doc string.

* src/fileio.c (Ffile_name_as_directory): Be precise about the
conditions under which a slash is appended.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 66ea761ca56..37948bc0ee9 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -508,7 +508,8 @@ This operation exists because a directory is also a file, but its name as
 a directory is different from its name as a file.
 The result can be used as the value of `default-directory'
 or passed as second argument to `expand-file-name'.
-For a Unix-syntax file name, just appends a slash.  */)
+For a Unix-syntax file name, just appends a slash unless a trailing slash
+is already present.  */)
   (Lisp_Object file)
 {
   char *buf;