RFR: 8248383: Clarify java.io.Reader.read(char[], ...) behavior for full array

[Brian Burkhalter]

Please review this small verbiage change to specify clearly the behavior of `Reader::read(char[] cbuf)` when the length of `cbuf` is zero, and that of `Reader::read(char[] cbuf, int off, int len)` when `len` is zero.

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Commit messages:
 - 8248383: Clarify java.io.Reader.read(char[], ...) behavior for full array

Changes: https://git.openjdk.java.net/jdk/pull/1737/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1737&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248383
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1737.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1737/head:pull/1737

PR: https://git.openjdk.java.net/jdk/pull/1737