doc patch: missing tags in protocol.sgml

Dear hackers,

I found that in protocol.sgml, the parameter "streaming" is mentioned twice [1] but
both are not tagged. IIUC, "streaming" can be <varname> and "parallel" can be <literal>.
Also, the first "streaming" can have a link to the pgoutput's manual. Attached patch fixes like that.

Thanks Peter Smith and Noboru Saito to confirm the point.

[1]:
```
         The LSN of the abort operation, present only when streaming is set to parallel.
...
         Abort timestamp of the transaction, present only when streaming is set to
         parallel. The value is in number of microseconds since PostgreSQL epoch (2000-01-01).
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED