Address feedback on platform-provided behaviors explainer (4)

[anaskim]

Updates

• 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 user-defined behaviors:
  • Example of a user-defined behavior
  • Example of pollyfill
  • Expanded considerations
• Some nits and added links to further clarify user research.