curl.1: mark the argument to --cookie as <data|filename>

From a discussion in #3676

Suggested-by: Tim Rühsen

Closes #3682

docs/cmdline-opts/cookie.d

@@ -1,6 +1,6 @@
 Short: b
 Long: cookie
-Arg: <data>
+Arg: <data|filename>
 Protocols: HTTP
 Help: Send cookies from string/file
 ---

src/tool_help.c

@@ -80,7 +80,7 @@ static const struct helptxt helptext[] = {
    "Connect to host"},
   {"-C, --continue-at <offset>",
    "Resumed transfer offset"},
-  {"-b, --cookie <data>",
+  {"-b, --cookie <data|filename>",
    "Send cookies from string/file"},
   {"-c, --cookie-jar <filename>",
    "Write cookies to <filename> after operation"},