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

[Naoto Sato]

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

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

Commit messages:
 - 8276188: Clarify "default charset" descriptions in String class

Changes: https://git.openjdk.java.net/jdk/pull/6198/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6198&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276188
  Stats: 16 lines in 1 file changed: 2 ins; 2 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6198.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6198/head:pull/6198

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