doc,test: clarify that Http2Stream is destroyed after data is read

Correct docs to clarify that behaviour,
and fix a race condition in test-http2-large-write-destroy.js.

Fixes: https://github.com/nodejs/node/issues/27863

Backport-PR-URL: https://github.com/nodejs/node/pull/28904
PR-URL: https://github.com/nodejs/node/pull/27891


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>