Document gitlab_pm db handle
Problem to solve
The gitlab_pm database handle was created as part of Replace license-finder MVC (&8072 - closed) and was meant to make package_metadata more scalable.
During some work by grouptenant scale the issue of this handle and lack of documentation about it came up #413543 (comment 1436897781)
For future maintainability and general knowledge, the gitlab_pm handle and related tables should be documented.
Proposal
Update https://docs.gitlab.com/ee/development/database/multiple_databases.html and any other necessary docs with info about gitlab_pm.