Document that `JsonReader` methods throw `IllegalStateException`

[Marcono1234]

Purpose

Resolves #2663

Description

As mentioned in #2663 and the comments, the JsonReader documentation currently does not mention for all methods that an IllegalStateException is thrown for mismatching JSON tokens.

This pull request adds the missing documentation, adjusts the existing documentation to be consistent, and also mentions for close() that subsequent use will cause an IllegalStateException.

Checklist

• [x] New code follows the Google Java Style Guide\ This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.\ Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• [ ] 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
• [x] mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

Thanks!