RFR: 8303077: JFR: Add example usage to jdk.jfr

[Erik Gahlin]

8303077: JFR: Add example usage to jdk.jfr

-------------

Commit messages:
 - Initial
 - Updates
 - Remove empty line
 - Initial

Changes: https://git.openjdk.org/jdk/pull/12718/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303077
  Stats: 302 lines in 8 files changed: 275 ins; 1 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/12718.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12718/head:pull/12718

PR: https://git.openjdk.org/jdk/pull/12718