Document limitations of progress reporting with InnoDB online DDL

When users read the existing documentation of the progress reporting feature, they usually expect their ALTER TABLE command to provide very good progress reporting of online DDL of InnoDB tables, because the documentation doesn't mention much about the limitations. However, there are some limitations about what progress is reported with InnoDB online DDL. As an example, see the comments of MDEV-8471.

I added the following section to the KB page, but I'm not sure if it's 100% correct and complete:

https://mariadb.com/kb/en/mariadb/progress-reporting/#limitations