Add a note about the necessity to restart pods

What does this MR do?

After configuring GitLab Helm chart with an external database, I noticed that some pods were throwing errors referring to the old database:

2022-02-16T12:40:15.549Z pid=21 tid=61nd ERROR: CRON JOB: could not translate host name "gitlab-postgresql.gitlab-ns.svc" to address: Name or service not known

It was necessary to restart all the pods to make GitLab Helm chart work with the new database.

This MR adds a note about it to the docs.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened