RFR: 8327991: Improve HttpClient documentation with regard to reclaiming resources

[Daniel Fuchs]

Please find here a doc-only change that tries to better document how resources allocated by the HttpClient are reclaimed.
The `@implNote` in the HttpClient class level API documentation is extended to document what happens if you don't explicitly close the HttpClient. The note is refactored to introduce the notion of streaming or publishing bodies. BodySubscribers and BodyHandlers now have links that link back to the HttpClient API note, as well as methods that return publishing or streaming body implementations.

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Commit messages:
 - add more information to HttpClient implNote
 - 8327991

Changes: https://git.openjdk.org/jdk/pull/18270/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18270&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327991
  Stats: 105 lines in 2 files changed: 79 ins; 9 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/18270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18270/head:pull/18270

PR: https://git.openjdk.org/jdk/pull/18270