Follow-up from "Update policy docs"

The following discussions from !26993 (merged) should be addressed:

• @eread started a discussion:

  The GitLab handbook documents [when something is a bug and when it is a feature request](https://about.gitlab.com/handbook/product/product-management/process/feature-or-bug.html).

• @eread started a discussion:

  ### Upgrades from version 12
  
  From version 12 onwards, an additional step is required. More significant migrations
  may occur during major release upgrades.
  
  To ensure these are successful:
  
  1. Increment to the first minor version (`x.0.x`) during the major version jump.
  1. Proceed with upgrading to a newer release.

• @eread started a discussion:

  | 12.5.8                | 11.3.4       | `11.3.4` -> `11.11.8` -> `12.0.12` -> `12.5.8`            | `11.11.8` is the last version in version `11`. `12.0.x` [is a required step](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/23211#note_272842444). |
  | 12.8.5                | 9.2.6       | `9.2.6` -> `9.5.10` -> `10.8.7` -> `11.11.8` -> `12.0.12` -> `12.8.5` | Four intermediate versions are required: the final 9.5, 10.8, 11.11 releases, plus 12.0. |