Platform provided behaviors feedback 5 (#1263)

# Updates
- API change: use `behaviors` everywhere (no `behaviorList`).
- Making behaviors unmutable. Also added open question on supporting dynamic updates in the future if valid use cases arise.
- Updating "Duplicates" section to match new API.
- Added primitives alternative.
- Added decorators alternative.
- Clarifying why we're starting with submit behavior.
- Nits.

Author: anaskim