Add documentation about ALTER TABLE .. RENAME COLUMN IF EXISTS

This useful statement works in MariaDB:

create table t (i int);

alter table if exists t rename column if exists i to j;

But the IF EXISTS clause isn't documented here:
https://mariadb.com/kb/en/alter-table/

The clause is documented for CHANGE COLUMN, but that's a less useful statement, given that the data type must be supplied as well.