swift234

[docs] Modernize the Access Control doc.

- Add a note about protocol extension members being used for conformances.
- Phrase in present tense.
- Remove references to "Swift 1.0".
- Replace "accessibility" with "access level". "Accessibility" has another
  meaning for app developers, so we avoid the term now in the compiler.
- 'required' and 'final' are now modifiers.
- 'internal' declarations don't show up in generated headers for frameworks.
- Remove the section on unit tests. We now have @testable.
- Remove the section on transparent functions, which isn't fully designed.

Swift SVN r31858