RFR: 8214561: Use {@systemProperty} for definition of "java.util.prefs.PreferencesFactory" system property

[Brent Christian]

Using the {@systemProperty} tag for the "java.util.prefs.PreferencesFactory" system property will allow it to be found in the main javadoc search index.

The updated doc build looks good.

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Commit messages:
 - 8214561: Use {@systemProperty} for definition of java.util.prefs.PreferencesFactory system property

Changes: https://git.openjdk.java.net/jdk/pull/965/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=965&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8214561
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/965.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/965/head:pull/965

PR: https://git.openjdk.java.net/jdk/pull/965