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

[Joe Wang]

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

Marked as reviewed by joehw (Reviewer).

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