Document deprecated Resources fields

## Issue

Nomad deprecated various Node and Allocation Resources fields around v0.9. Since our API docs lack per-response-field level of detail, the deprecations have gone undocumented outside of code.

- Alloc fields: https://github.com/hashicorp/nomad/blob/v0.12.5/nomad/structs/structs.go#L8538-L8555
- Node fields: https://github.com/hashicorp/nomad/blob/v0.12.5/nomad/structs/structs.go#L1745-L1748

## Resolution 1: Upgrade guide

Dig into git and retcon the upgrade specific guide to mention these deprecations: https://www.nomadproject.io/docs/upgrade/upgrade-specific

## Resolution 2: Document response fields in API docs

Ideally our API docs would include per-response-field information, but this is a large effort and should not block more immediate solutions to the deprecation issue.

Start with the deprecated Allocation and Node Resources fields in nomad/structs/structs.go at the linked ranges, then review the git history for their deprecation context. Update docs/upgrade/upgrade-specific with the relevant deprecations and verify that the guide clearly identifies the affected fields and versions.

Written by the indexing model from the issue text.