Doc: Clarify where JavaScript fits in QML code

- Rewrite the introductory list.

- Rewrite the section about application startup code.

- Flatten sections. Previously, there were two \section1 headings:
  "JavaScript Expressions in QML Objects" and "Running JavaScript at
  Startup". This split wasn't very natural, as startup code needs to be
  called from an object, while imported JS files aren't in objects.

- Some minor formatting/wording changes in unrelated areas.

Change-Id: Ie08f94e2ea77ca84f80491c782bc30612f7ef698
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>