ls: Improve description of --group-directories-first.

* src/ls.c (usage) [--group-directories-first]: Improve description.
* doc/coreutils.texi (Which files are listed): Likewise.

doc/coreutils.texi

@@ -5894,6 +5894,8 @@ directories and the files separately using the selected sort key
 (see --sort option).
 That is, this option specifies a primary sort key,
 and the --sort option specifies a secondary key.
+However, any use of @option{--sort=none}
+(@option{-U}) disables this option altogether.
 
 @item --hide=PATTERN
 @opindex --hide=@var{pattern}

src/ls.c

@@ -4350,7 +4350,9 @@ Mandatory arguments to long options are mandatory for short options too.\n\
 "), stdout);
       fputs (_("\
       --group-directories-first\n\
-                             group directories before files\n\
+                             group directories before files.\n\
+                               augment with a --sort option, but any\n\
+                               use of --sort=none (-U) disables grouping\n\
 "), stdout);
       fputs (_("\
   -G, --no-group             in a long listing, don't print group names\n\