chore: document subgraph schema

[banasa44]

Description

2nd part of the inline comments for subgraph schema. These are meant to be reflected in our auto-generated documentation (subgraph/reference-guide) in our developer-portal.

https://subgraph.satsuma-prod.com/aragon/osx-carles-sepolia/playground

Task ID: OS-1206

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have selected the correct base branch.
• [ ] I have performed a self-review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] My changes generate no new warnings.
• [ ] Any dependent changes have been merged and published in downstream modules.
• [ ] I ran all tests with success and extended them if necessary.
• [ ] I have updated the CHANGELOG.md file in the root folder.
• [ ] I have updated the DEPLOYMENT_CHECKLIST file in the root folder.
• [ ] I have updated the UPDATE_CHECKLIST file in the root folder.
• [x] I have updated the Subgraph and added a QA URL to the description of this PR.