Docs: update stream error FAQ

[D4ryl00]

Hi!
I encountered an error on a server stream call that is not well documented. The full discussion is here: connectrpc/connect-go#757

To summarize, the WriteTimeout on the http.Server caused me this error after the timeout: invalid_argument: protocol error: incomplete envelope: unexpected.
Should we update the documentation here to mention that?

Thanks.

[D4ryl00]

Or may connect-go be fixed to return a more appropriate error? In this case, should I open a PR in the connect-go repository?