Document binary log checkpoints

Binary log checkpoints were added as part of MDEV-181. However, I don't see much information about them on the following page:

https://mariadb.com/kb/en/library/overview-of-the-binary-log/

There is some detailed information about the new feature on MDEV-181. One unclear point about that description is that when it refers to "XA transactions", is it referring to internal XA transactions or just external XA transactions? As the MySQL documentation points out, XA-capable storage engines also use internal XA transactions in some cases, even if the application is not explicitly using XA.

https://dev.mysql.com/doc/refman/5.6/en/xa-restrictions.html