resolver.3: Mention that some functions set 'h_errno'

Reported-by: Wladimir Mutel <muwlgr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/resolver.3

@@ -461,6 +461,20 @@ and
 .BR dn_expand ()
 functions return the length
 of the compressed name, or \-1 if an error occurs.
+.PP
+In the case of an error return from
+.BR res_nquery (),
+.BR res_query (),
+.BR res_nsearch (),
+.BR res_search (),
+.BR res_nquerydomain (),
+or
+.BR res_querydomain (),
+the global variable
+.I h_errno
+(see
+.BR gethostbyname (3))
+can be consulted to determine the cause of the error.
 .SH FILES
 .TP
 .I /etc/resolv.conf