doc: restore documentation for two error codes

ERR_HTTP2_ERROR and ERR_UNKNOWN_BUILTIN_MODULE error codes documentation
seem to have been accidentally removed in commit
1cdb41f2 (pull request #15160).

This reverts that removal, restoring the documentation for those two
error codes.

Those error codes are used from lib/ folder.

This is a part of the fixes hinted by #21470, which includes some tests
for error codes usage and documentation and enforces a stricter format.

PR-URL: https://github.com/nodejs/node/pull/21484
Refs: https://github.com/nodejs/node/pull/21470
Refs: https://github.com/nodejs/node/issues/21440
Refs: https://github.com/nodejs/node/pull/15160


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>