Improve documentation about default JsonReader and JsonWriter behavior

[Marcono1234]

Purpose

Tries to improve the documentation about default JsonReader and JsonWriter behavior, and related TypeAdapter methods

Description

The current default behavior was not explicitly mentioned; also for the TypeAdapter methods it was not clear that their behavior differs from the default Gson behavior, see also #1133.

Also removes some empty lines at the start of <pre> blocks in the documentation (they also appeared in the rendered HTML).

Checklist

• New code follows the Google Java Style Guide
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors