DiG: document +qid=<num> option in the usage summary

The +qid=<num> option, which sets the outgoing query ID, was missing
from the usage summary printed using `dig -h` command.

bin/dig/dig.c

@@ -259,6 +259,8 @@ help(void) {
 	       "request)\n"
 	       "                 +padding=###        (Set padding block size "
 	       "[0])\n"
+	       "                 +qid=###            (Specify the query ID to "
+	       "use when sending queries)\n"
 	       "                 +[no]qr             (Print question before "
 	       "sending)\n"
 	       "                 +[no]question       (Control display of "