RFR: JDK-8319122: Improve documentation of various Zip-file related APIs

[Yakov Shafranovich]

The various Zip/Jar-file related Java APIs have some long-standing differences or peculiarities with respect to the ZIP-file specification or compared to other implementations which should be documented in the API-doc. This documents the following:
- Cache of JAR files in JarURLConnection class
- Cache of JAR/ZIP files in JarFile and ZipFile classes
- Unexpected behavior when parsing ZIP files with duplicate entries in JarFile and ZipFile classes, as well as the zipfs provider
- Directories and filenames with the same name considered to be the same in ZipFile class
- Possible issues when local and central headers conflict in ZipInputStream class

Related JBS report:
https://bugs.openjdk.org/browse/JDK-8319122

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Commit messages:
 - document patch for JDK-8319122

Changes: https://git.openjdk.org/jdk/pull/16424/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319122
  Stats: 41 lines in 5 files changed: 38 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16424.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16424/head:pull/16424

PR: https://git.openjdk.org/jdk/pull/16424