Geo Zero-downtime updates: Rework docs so post-deploy migrations are run *after* updating secondary nodes

Noted by @stanhu in https://gitlab.com/gitlab-org/gitlab-ee/issues/13009#note_198417737:

Problem

If I'm looking at update steps for a HA set up they say:

• update GitLab on deploy node (but do not run post-deploy migrations)
• update GitLab on all other nodes
• run post-deploy migrations on deploy node

This procedure will ensure all nodes run the new version and will be compatible with the changes that will be applied by the post-deploy migrations.

To apply a similar scenario to Geo:

• update primary (no post-deploy migrations)
• update all secondaries
• run post-deploy migrations on primary

But that contridicts current Geo docs: https://docs.gitlab.com/omnibus/update/README.html#geo-deployment

Suggestion

Instruct users to update Geo secondary nodes before running the post-deploy migrations.