http2,doc: minor fixes

Some small fixes on HTTP/2 and its documentation:

• Fix server side example: start data flow when receiving a stream 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, an empty DATA frame will always be sent, because the streams layer waits until data has been flushed before queueing EOF)

• Fix debug message

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines