RFR: 6587699: Document DigestInputStream behavior when skip() or mark() / reset() is used [v3]

[Weijun Wang]

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

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  rewording

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9667/files
  - new: https://git.openjdk.org/jdk/pull/9667/files/a6304352..36e46dfe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9667&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9667&range=01-02

  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 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