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
1cdb41f (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: #21484
Refs: #21470
Refs: #21440
Refs: #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>

doc/api/errors.md

@@ -869,6 +869,11 @@ forbidden.
 For HTTP/2 requests using the `CONNECT` method, the `:scheme` pseudo-header is
 forbidden.
 
+<a id="ERR_HTTP2_ERROR"></a>
+### ERR_HTTP2_ERROR
+
+A non-specific HTTP/2 error has occurred.
+
 <a id="ERR_HTTP2_GOAWAY_SESSION"></a>
 ### ERR_HTTP2_GOAWAY_SESSION
 
@@ -1644,6 +1649,13 @@ A string that contained unescaped characters was received.
 An unhandled error occurred (for instance, when an `'error'` event is emitted
 by an [`EventEmitter`][] but an `'error'` handler is not registered).
 
+<a id="ERR_UNKNOWN_BUILTIN_MODULE"></a>
+### ERR_UNKNOWN_BUILTIN_MODULE
+
+Used to identify a specific kind of internal Node.js error that should not
+typically be triggered by user code. Instances of this error point to an
+internal bug within the Node.js binary itself.
+
 <a id="ERR_UNKNOWN_CREDENTIAL"></a>
 ### ERR_UNKNOWN_CREDENTIAL