curl.1: --trace supports % for sending to stderr!

docs/curl.1

@@ -1818,7 +1818,8 @@ algorithms curl supports, and uncompress the data while receiving it.
 .IP "--trace <file>"
 Enables a full trace dump of all incoming and outgoing data, including
 descriptive information, to the given output file. Use "-" as filename to have
-the output sent to stdout.
+the output sent to stdout. Use "%" as filename to have the output sent to
+stderr.
 
 This option overrides previous uses of \fI-v, --verbose\fP or
 \fI--trace-ascii\fP.