err: document ABORT_ERR code

We added an `AbortError` with the same code and name as the web's as
part of the consensus in #36084 but
never actually documented the error in our error codes list.

This PR adds the error code.

doc/api/errors.md

@@ -612,6 +612,18 @@ A human-readable string describing the reason for the error.
 <a id="nodejs-error-codes"></a>
 ## Node.js error codes
 
+<a id="ABORT_ERR"></a>
+### `ABORT_ERR`
+<!-- YAML
+added: v15.0.0
+-->
+Used when an operation has been aborted (typically using an `AbortController`).
+
+APIs _not_ using `AbortSignal`s typically do not raise an error with this code.
+
+This code does not use the regular `ERR_*` convention Node.js errors use in
+order to be compatible with the web platform's `AbortError`.
+
 <a id="ERR_AMBIGUOUS_ARGUMENT"></a>
 ### `ERR_AMBIGUOUS_ARGUMENT`