doc: clarify http2 server.close() behavior
This commit is an attempt to clarify the behavior of HTTP2's server.close()
method. Specifically, this makes it more clear that the server stops allowing new sessions although the callback may not be invoked for some time due to previously existing sessions.
Fixes: https://github.com/nodejs/node/issues/28214
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines