Document truncate_legacy_tables rake task

The following rake tasks need to be documented:

• rake gitlab:db:truncate_legacy_tables:main
• rake gitlab:db:truncate_legacy_tables:ci

MR introducing the rake tasks !94252 (merged)

Can be added at the end of this section: https://docs.gitlab.com/ee/development/database/multiple_databases.html#locking-writes-on-the-tables-that-dont-belong-to-the-database-schemas