doc: simplify http2 wording and formatting

Remove `It is important to note that` and italics from `waitForTrailers`
sentences.

PR-URL: https://github.com/nodejs/node/pull/22541


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>