Change the dig documentation

(cherry picked from commit 7697923eab)

bin/dig/dig.c

@@ -193,8 +193,10 @@ help(void) {
 "                 +[no]fail           (Don't try next server on SERVFAIL)\n"
 "                 +[no]header-only    (Send query without a question section)\n"
 "                 +[no]identify       (ID responders in short answers)\n"
-"                 +[no]idnin          (Parse IDN names)\n"
-"                 +[no]idnout         (Convert IDN response)\n"
+#ifdef HAVE_LIBIDN2
+"                 +[no]idnin          (Parse IDN names [default=on on tty])\n"
+"                 +[no]idnout         (Convert IDN response [default=on on tty])\n"
+#endif
 "                 +[no]ignore         (Don't revert to TCP for TC responses.)\n"
 "                 +[no]keepopen       (Keep the TCP socket open between queries)\n"
 "                 +[no]mapped         (Allow mapped IPv4 over IPv6)\n"

bin/dig/dig.docbook

@@ -789,7 +789,13 @@
 	    <para>
 	      Process [do not process] IDN domain names on input.
 	      This requires IDN SUPPORT to have been enabled at
-	      compile time.  The default is to process IDN input.
+	      compile time.
+	    </para>
+	    <para>
+	      The default is to process IDN input when standard output
+	      is a tty.  The IDN processing on input is disabled when
+	      dig output is redirected to files, pipes, and other
+	      non-tty file descriptors.
 	    </para>
 	  </listitem>
 	</varlistentry>
@@ -800,7 +806,13 @@
 	    <para>
 	      Convert [do not convert] puny code on output.
 	      This requires IDN SUPPORT to have been enabled at
-	      compile time.  The default is to convert output.
+	      compile time.
+	    </para>
+	    <para>
+	      The default is to process puny code on output when
+	      standard output is a tty.  The puny code processing on
+	      output is disabled when dig output is redirected to
+	      files, pipes, and other non-tty file descriptors.
 	    </para>
 	  </listitem>
 	</varlistentry>