Address feedback on platform-provided behaviors explainer (4) (#1251)

- Added section to clarify addition of duplicate behaviors.
- Added section to discuss API naming, also added a couple of alternatives to consider.
- Added section on composition and conflict resolution (deleted it from Future work and brought it up to the main proposal to explore this further).
- Added examples of compatible and conflicting behaviors, issues and resolution.
- Expanded the section of developer-defined behaviors:
  - Example of a developer-defined behavior
  - Example of pollyfill
  - Expanded considerations
- Some nits and added links to further clarify user research.

Author: anaskim