RFR: 8345577: Additional clarification to the com.sun.net.httpserver.HttpExchange.getAttribute() and setAttribute() methods after JDK-8235786

[Jaikiran Pai]

Can I please get a review of this doc-only change which proposes to address https://bugs.openjdk.org/browse/JDK-8345577?

As noted in that issue, a recent update (in the current JDK mainline) to the javadoc of these methods made some existing external subclasses overridding these methods to contradict the specification of the methods. The update in this current PR relaxes the normative text and clarifies the JDK built-in implementation through a `@implSpec`.

No new tests have been added given the nature of this change. The CSR is ready for review https://bugs.openjdk.org/browse/JDK-8345582.

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Commit messages:
 - 8345577: Additional clarification to the com.sun.net.httpserver.HttpExchange.getAttribute() and setAttribute() methods after JDK-8235786

Changes: https://git.openjdk.org/jdk/pull/22577/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22577&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345577
  Stats: 15 lines in 1 file changed: 11 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/22577.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22577/head:pull/22577

PR: https://git.openjdk.org/jdk/pull/22577