RFR: 8261301: StringWriter.flush() is NOOP but documentation does not indicate it

[Brian Burkhalter]

The specification of the method `flush()` in the `java.io` classes `CharArrayWriter` and `StringWriter` is not explicit about the fact that the method has no effect. This request proposes to add to the specification of each flush() method the sentence
The {@code flush} method of {@code <ClassName/>} does nothing.
The corresponding CSR is JDK-8264867.

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Commit messages:
 - 8261301: StringWriter.flush() is NOOP but documentation does not indicate it

Changes: https://git.openjdk.java.net/jdk/pull/3383/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3383&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261301
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3383.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3383/head:pull/3383

PR: https://git.openjdk.java.net/jdk/pull/3383