http2,doc: minor fixes

Some small fixes on HTTP/2 and its documentation:

 - Add a note that, on server streams, it's not necessary
   to start data flow.

 - Set EOF flag if we have marked all data for sending:
   there's no need to wait until the queue is
   actually empty (and send a separate, empty DATA).

   (Note that, even with this change, a separate DATA
   frame will always be sent, because the streams
   layer waits until data has been flushed before
   dispatching EOF)

PR-URL: https://github.com/nodejs/node/pull/28044


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>