manpage: add --protocol-decoder-jsontrace command line option

Support for JSON formatted decoder output was introduced in commit
5461491 ("decode: add support for Google Trace Event output (JSON)")
in 2020-07. Update the manpage.

doc/sigrok-cli.1

@@ -429,6 +429,10 @@ This allows consumers to receive machine readable timing information.
 .BR "\-\-protocol\-decoder\-ann\-class
 When given, decoder annotations will include annotation class names.
 .TP
+.BR "\-\-protocol\-decoder\-jsontrace
+When given, decoder output uses the Google Trace Event format (JSON).
+Which can be inspected in web browsers or other viewers.
+.TP
 .BR "\-l, \-\-loglevel " <level>
 Set the libsigrok and libsigrokdecode loglevel. At the moment \fBsigrok\-cli\fP
 doesn't support setting the two loglevels independently. The higher the