docs: add substantial (#891)

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<!-- 1. Explain WHAT the change is about -->

- docs

<!-- 2. Explain WHY the change cannot be made simpler -->

- N/A

<!-- 3. Explain HOW users should update their code -->

#### Migration 

N/A

- [ ] The change comes with new or modified tests
- [ ] Hard-to-understand functions have explanatory comments
- [ ] End-user documentation is updated to reflect the change