docs: clarify Cache Components runtime config (#99820)
## Summary
Clarify that Cache Components does not support the runtime route segment
config, including an explicit Node.js runtime value.
Document the required removal in the migration guide, runtime API
reference, and cacheComponents reference so the guidance matches the
build diagnostic.
## Verification
- Confirmed the existing runtime-edge migration anchor remains
unchanged.
- Not run: runtime tests (documentation-only change).
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