RFR: 8272334: com.sun.net.httpserver.HttpExchange: Improve API doc of getRequestHeaders

[Julia Boes]

This is a doc-only fix that improves the wording of the API doc of `getRequestHeaders()`. The other changes are trivial cleanup.

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Commit messages:
 - cleanup
 - Add note on multiple header fields
 - initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5100&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272334
  Stats: 27 lines in 1 file changed: 5 ins; 3 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5100.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5100/head:pull/5100

PR: https://git.openjdk.java.net/jdk/pull/5100