Improve documentation about increasing number of maximum connections

There is at least one report related to increase of maximum connections for two-connection setup. We need to improve visibility of https://docs.gitlab.com/omnibus/settings/database.html#configuring-multiple-database-connections

Possible options:

• We advise to run rake gitlab:check. Maybe integrate the maximum-connection check in that rake task
• Mention connection checking on https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html