chore(docs): note graphql relationship query limitations

[wkentdag]

Description

Updates the documentation for GraphQL queries to note one of its limitations compared to the local / rest APIs. As discussed in https://github.com/payloadcms/payload/discussions/761#discussioncomment-10399114

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Chore (non-breaking change which does not add functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Change to the templates directory (does not affect core functionality)
• [ ] Change to the examples directory (does not affect core functionality)
• [x] This change requires a documentation update

Checklist:

• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] Existing test suite passes locally with my changes
• [x] I have made corresponding changes to the documentation