Improve directory-append manual entry

* doc/lispref/files.texi (Directory Names): Mention zero-length
restriction.

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index a624c2eb937cc3e78b0026a42fa2847e73cb09ac..804cef292ee9af7225d4042690edd54a6fc543a1 100644 (file)
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -2355,6 +2355,8 @@ didn't end with a slash.
 @end group
 @end example
 
+A zero-length directory or component is not allowed.
+
 This is almost the same as using @code{concat}, but @var{dirname} (and
 the non-final components) may or may not end with slash characters,
 and this function will not double those characters.