docs: add missing Arg tag to --stderr

Prior to this change the required argument was not shown.

curl.1 before: --stderr
curl.1 after: --stderr <file>

curl --help before:
     --stderr        Where to redirect stderr

curl --help after:
     --stderr <file>  Where to redirect stderr

Closes https://github.com/curl/curl/pull/6692

docs/cmdline-opts/stderr.d

@@ -1,4 +1,5 @@
 Long: stderr
+Arg: <file>
 Help: Where to redirect stderr
 See-also: verbose silent
 Category: verbose

src/tool_help.c

@@ -742,7 +742,7 @@ static const struct helptxt helptext[] = {
   {"-3, --sslv3",
    "Use SSLv3",
    CURLHELP_TLS},
-  {"    --stderr",
+  {"    --stderr <file>",
    "Where to redirect stderr",
    CURLHELP_VERBOSE},
   {"    --styled-output",