RFR: 8222329: Readable read(CharBuffer) does not specify that 0 is returned when there is no remaining space in buffer

[Brian Burkhalter]

Clarify the behavior of `java.lang.Readable` when the specified `java.nio.CharBuffer` parameter is empty but read-only, and when it is full.

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Commit messages:
 - 8222329: Readable read(CharBuffer) does not specify that 0 is returned when there is no remaining space in buffer

Changes: https://git.openjdk.org/jdk/pull/14616/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14616&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8222329
  Stats: 17 lines in 3 files changed: 9 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14616/head:pull/14616

PR: https://git.openjdk.org/jdk/pull/14616