doc: add text about error.code stability

Fixes: nodejs#23975

PR-URL: nodejs#24090
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

doc/api/errors.md

@@ -260,7 +260,7 @@ not capture any frames.
 * {string}
 
 The `error.code` property is a string label that identifies the kind of error.
-See [Node.js Error Codes][] for details about specific codes.
+`error.code` is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, `error.message` strings may change between any versions of Node.js. See [Node.js Error Codes][] for details about specific codes.
 
 ### error.message