Platform provided behaviors feedback 6 (#1271)

# Updates
- `attributeChangedCallback` clarifications with examples
- Added section `Dynamically changing the type attribute`
- Deleted mentions of `behaviorDetachedCallback` as behaviors are no longer mutable
- Added PlatformBehavior API to describe what would be needed for developer-defined behaviors
- Open question on "behaviors" name
- PlatformBehavior -> ElementBehavior
- Nits: fixing spacing of examples

Author: anaskim