Docs: Document ES_CLASSPATH removal in breaking changes

Closes #14008

docs/reference/migration/migrate_2_0/settings.asciidoc

@@ -174,3 +174,9 @@ environment variable, or the `-Des.config`, `-Des.default.config`, or
 Instead, the config file must be named `elasticsearch.yml` and must be located
 in the default `config/` directory, or in the directory specified in the
 `CONF_DIR` environment variable.
+
+==== `ES_CLASSPATH removed`
+
+The `ES_CLASSPATH` environment variable is no longer used to set the class
+path. External libraries should preferably be loaded using the plugin
+mechanism or, if you really must, be copied to the `lib/` directory.