RFR: 8276188: Clarify "default charset" descriptions in String class

[Iris Clark]

On Mon, 1 Nov 2021 18:31:17 GMT, Naoto Sato <naoto at openjdk.org> wrote:

> This is a leftover document fix to `String` class for the JEP 400. Corresponding CSR has also been drafted: https://bugs.openjdk.java.net/browse/JDK-8276238

Diffs align with the corresponding CSR, which I have also reviewed.

-------------

Marked as reviewed by iris (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6198