docs(fdw): correct relationships table caveat columns

[capybara-alt]

Description

Fixes incorrect caveat column names and types in internal/fdw/README.md.

The documentation currently describes:

• optional_caveat_name (text)
• optional_caveat_context (jsonb)

However, the implementation and the example configuration use:

• caveat_name (text)
• caveat_context (text)

This PR updates the documentation to match the current implementation.

Testing

Verified that the documentation matches:

• internal/fdw/tables/relationships.go
• internal/fdw/configuration.sql

Documentation change only. No functional changes.

References

Closes #3227

[github-actions]

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