RDS/Aurora Docs clarifications.

Hi,

1. In the [RDS docs](https://github.com/github/gh-ost/blob/master/doc/rds.md), a limitation with `binlog_format` and pt-table-checksum is referenced. In Aurora MySQL version 3, users are able to set `binlog_format` at the session level natively. Could this information regarding aurora version 3 be added?
```
mysql> select @@version,@@aurora_version,@@binlog_format;
+-----------+------------------+-----------------+
| @@version | @@aurora_version | @@binlog_format |
+-----------+------------------+-----------------+
| 8.0.32 | 3.05.2 | STATEMENT |
+-----------+------------------+-----------------+
1 row in set (0.00 sec)

mysql> set session binlog_format='row';
Query OK, 0 rows affected (0.01 sec)

mysql> select @@version,@@aurora_version,@@binlog_format;
+-----------+------------------+-----------------+
| @@version | @@aurora_version | @@binlog_format |
+-----------+------------------+-----------------+
| 8.0.32 | 3.05.2 | ROW |
+-----------+------------------+-----------------+
1 row in set (0.00 sec)
```

2.
> Aurora does not allow editing of the read_only parameter. While it is defined as {TrueIfReplica}, the parameter is non-modifiable field.

`read_only` can be modified in cluster parameter groups and from Aurora MySQL version 3.06 and higher via the `mysql.rds_set_read_only` stored procedure([details here](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/mysql-stored-proc-replicating.html#mysql_rds_set_read_only) ). Only caveat is reader instances are always set to `innodb_read_only=1` which cannot be modified.

Thanks!
Marc