RFR: 8215788: Clarify JarInputStream Manifest access

[Lance Andersen]

Please review this PR which updates  the JarInputStream class description to clarify when the Manifest is accessible via JarInputStream::getManifest and JarInputStream::get[Jar]Entry.

It is worth noting that with this update, we are finally documenting  behavior that dates back to when this class was added to JDK 1.2


Best,
Lance

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Commit messages:
 - switch linkePlain to code tag
 - Addressed 3rd draft comments
 - Merge branch 'master' into JDK-8215788
 - Third draft of the class description
 - Minor update to the wording for signature entries
 - Inital draft to clarify Manifest access

Changes: https://git.openjdk.org/jdk/pull/10045/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10045&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8215788
  Stats: 47 lines in 1 file changed: 41 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10045.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10045/head:pull/10045

PR: https://git.openjdk.org/jdk/pull/10045