doc: shred: mention unsupported file types

* src/shred.c (usage): Mention the behavior of 'shred' on FIFOs,
sockets, and terminals.
* doc/coreutils.texi (shred invocation): Likewise.
Addresses https://bugs.gnu.org/30177

doc/coreutils.texi

@@ -10372,6 +10372,10 @@ to destroy using @command{shred}, be sure that it is not backed up or mirrored.
 shred [@var{option}]@dots{} @var{file}[@dots{}]
 @end example
 
+If @var{file} is a FIFO, socket, or terminal, @command{shred} skips it
+and emit a diagnostic message because the contents of these file types
+do not reside on disk.
+
 The program accepts the following options.  Also see @ref{Common options}.
 
 @table @samp

src/shred.c

@@ -220,6 +220,11 @@ If FILE is -, shred standard output.\n\
       oputs (VERSION_OPTION_DESCRIPTION);
       fputs (_("\
 \n\
+FILE will be skipped with a diagnostic message if it is a FIFO, socket, or\n\
+terminal, since its data does not reside on disk.\n\
+"), stdout);
+      fputs (_("\
+\n\
 Delete FILE(s) if --remove (-u) is specified.  The default is not to remove\n\
 the files because it is common to operate on device files like /dev/hda,\n\
 and those files usually should not be removed.\n\