RFR: 8235786: Javadoc for com/sun/net/httpserver/HttpExchange.java#setAttribute is unclear

[Jaikiran Pai]

Can I please get a review of this doc-only change which proposes to clarify the behaviour of `com.sun.net.httpserver.HttpExchange.setAttribute()` and `com.sun.net.httpserver.HttpExchange.getAttribute()` methods?

As noted in https://bugs.openjdk.org/browse/JDK-8235786, it's not clear from the javadoc of these methods that the attributes that are set or retrieved through the exchange are actually those that belong to the entire `com.sun.net.httpserver.HttpContext` and thus are shared by all exchanges.

The commit in this PR specifies this behaviour to match the current implementation. I'll file a CSR once we settle on the text.

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Commit messages:
 - 8235786: Javadoc for com/sun/net/httpserver/HttpExchange.java#setAttribute is unclear

Changes: https://git.openjdk.org/jdk/pull/22454/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22454&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8235786
  Stats: 13 lines in 1 file changed: 7 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22454.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22454/head:pull/22454

PR: https://git.openjdk.org/jdk/pull/22454