[Maria-docs] minor feature not mentioned in pbxt docs

PBXT supports FOREIGN KEYS. When a row is deleted/updated in the parent table, InnoDB (and XtraDB) may perform one of the following actions: RESTRICT | CASCADE | SET NULL | NO ACTION The action "SET DEFAULT" (which is standard SQL) is not supported by InnoDB, but it's supported by PBXT. I think that this should be mentioned by the documentation. Also, maybe it should be stated that if the parent table is InnoDB the child table can't be PBXT and vice versa. Federico