RFR: 6587699: Document DigestInputStream behavior when skip() or mark() / reset() is used

[Weijun Wang]

Add an `@implNote` to clarify the behavior when these methods are called. A new test is added.

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Commit messages:
 - the implNote and new test

Changes: https://git.openjdk.org/jdk/pull/9667/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9667&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6587699
  Stats: 75 lines in 2 files changed: 75 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9667.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9667/head:pull/9667

PR: https://git.openjdk.org/jdk/pull/9667