Document the use of database `disallow_database_ddl_feature_flags` FF

Leonardo da Rosa - OOO until Nov. 21requested to merge
417161-document-use-of-disallow-database-ddl-feature-flags into master

Documents the usage of disallow_database_ddl_feature_flags during database upgrade

Related to gitlab-org/gitlab#417161 (closed)

Edited by Leonardo da Rosa - OOO until Nov. 21

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