uniq.1: add [input [output]] information

uniq.1

@@ -13,14 +13,20 @@
 .Op Ar input Op Ar output
 .Sh DESCRIPTION
 .Nm
-reads
-.Ar file
-and writes one copy of a line from each group of consecutive
-duplicate lines to stdout. If no
-.Ar file
-is given,
+reads the
+.Ar input
+file and writes one copy of a line from each group of consecutive
+duplicate lines to the
+.Ar output
+file. If no
+.Ar input
+file is given,
 .Nm
-reads from stdin.
+reads from stdin. If no
+.Ar output
+file is given, then
+.Nm
+writes to stdout.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl c