cdc: Document __crdb__ tag in avro schema

Update https://www.cockroachlabs.com/docs/v23.1/changefeed-messages documentation
on avro schema changes: we currently indicate:

```
Note that the original CockroachDB column definition is also included in the schema as a doc field, so it's still possible to distinguish between a NOT NULL CockroachDB column and a NULL CockroachDB column.
```
We should say that schema validation tools should *ignore* this field -- this is an internal cockroachDB specific schema type description that may change between cockroach versions; and as such it should be consider custom attribute that should be ignored.

Jira issue: CRDB-33426