Document return codes of dig

The dig tool reports some states as exit status. Document them briefly
in the manual page.

bin/dig/dig.rst

@@ -647,6 +647,26 @@ To turn off IDN support, use the parameters
 ``+noidnin`` and ``+noidnout``, or define the ``IDN_DISABLE`` environment
 variable.
 
+Return Codes
+~~~~~~~~~~~~
+
+``dig`` return codes are:
+
+``0``
+   DNS response received, including NXDOMAIN status
+
+``1``
+   Usage error
+
+``8``
+   Couldn't open batch file
+
+``9``
+   No reply from server
+
+``10``
+   Internal error
+
 Files
 ~~~~~
 

doc/man/dig.1in

@@ -661,6 +661,26 @@ a request to a DNS server or displaying a reply from the server.
 To turn off IDN support, use the parameters
 \fB+noidnin\fP and \fB+noidnout\fP, or define the \fBIDN_DISABLE\fP environment
 variable.
+.SH RETURN CODES
+.sp
+\fBdig\fP return codes are:
+.INDENT 0.0
+.TP
+.B \fB0\fP
+DNS response received, including NXDOMAIN status
+.TP
+.B \fB1\fP
+Usage error
+.TP
+.B \fB8\fP
+Couldn\(aqt open batch file
+.TP
+.B \fB9\fP
+No reply from server
+.TP
+.B \fB10\fP
+Internal error
+.UNINDENT
 .SH FILES
 .sp
 \fB/etc/resolv.conf\fP