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

[Bradford Wetmore]

On Wed, 27 Jul 2022 23:02:03 GMT, Weijun Wang <weijun at openjdk.org> wrote:

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

Be sure to update the commit message.  :)  

Hope to look at the rest later.

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