RFR: 8297686 JFR: Improve documentation of EventStream::onMetadata(Consumer)

[Erik Gahlin]

Could I have review of this docs fix.

Testing: make docs + test/jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Merge branch 'master' into meta2
 - Initial

Changes: https://git.openjdk.org/jdk/pull/11381/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11381&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297686
  Stats: 40 lines in 2 files changed: 39 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11381.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11381/head:pull/11381

PR: https://git.openjdk.org/jdk/pull/11381