doc,http: indicate callback is optional for message.setTimeout()

Documents that callback is optional for IncomingMessage setTimeout

PR-URL: #29654
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: trivikr

doc/api/http.md

@@ -1778,7 +1778,7 @@ added: v0.11.6
 The raw request/response trailer keys and values exactly as they were
 received. Only populated at the `'end'` event.
 
-### message.setTimeout(msecs, callback)
+### message.setTimeout(msecs[, callback])
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