Summary:
Improve migrations README. Specifically, add notes related to:
- pinned objects, and
- backport of scripts to PG11.2-based YB branches.
Jira: DB-13234

Test Plan: jenkins: skip

Reviewers: fizaa

Reviewed By: fizaa

Subscribers: tfoucher, mihnea, yql

Differential Revision: https://phorge.dev.yugabyte.com/D38820