document the --output in the man page and help text

docs/pv.1

@@ -344,6 +344,11 @@ invocations in a single, long, pipeline.
 
 .SH DATA TRANSFER MODIFIERS
 .TP
+.BI \-o\  FILE \fR,\ \fB\-\-output\  FILE
+Write data to
+.I FILE
+rather than stdout.
+.TP
 .BI \-L\  RATE \fR,\ \fB\-\-rate-limit\  RATE
 Limit the transfer to a maximum of
 .I RATE

src/main/help.c

@@ -360,6 +360,9 @@ void display_help(void)
 		 N_("use cursor positioning escape sequences"),
 		 { 0, 0, 0, 0} },
 		{ "", NULL, NULL, NULL, { 0, 0, 0, 0} },
+		{ "-o", "--output", N_("FILE"),
+		 N_("write output to FILE instead of stdout"),
+		 { 0, 0, 0, 0} },
 		{ "-L", "--rate-limit", N_("RATE"),
 		 N_("limit transfer to RATE bytes per second"),
 		 { 0, 0, 0, 0} },