Document how to check connections using rake task
This is a follow up for #390204 (closed)
In MR !111927 (merged), we added a rake task rake gitlab:db:decomposition:connection_status
for checking current PostgreSQL connection status.
Administrators of self-managed GitLab instances should be encouraged to verify their connection settings before they enable two connections
For example here: https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html