Guarantee that JsonElement.toString() produces JSON

[Marcono1234]

(Depends on #2654)

Purpose

Resolves #591
Resolves #1439

Description

As pointed out in #591 the result of JsonElement.toString() is currently unspecified. However, many users and also Gson's own unit tests assume that the output is valid JSON.

Changing the JsonElement.toString() behavior would likely be backward incompatible. Therefore it seems reasonable to guarantee in the documentation that toString() produces JSON, which is already the case implementation-wise.

Checklist

• 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
• mvn clean verify javadoc:jar passes without errors