RFR: 8299836: Make `user.timezone` system property searchable

[Justin Lu]

The system property _user.timezone_ is specified in the _TimeZone.getDefault()_ and _TimeZone.setDefault()_  methods. The javadoc search box should be able to match on the system property name.

This change replaces the **@code** tag with the **@systemProperty** tag for instances of _user.timezone_ when appropriate.

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Commit messages:
 - Fix setDefault()
 - second user.timezone fix
 - Switch tag

Changes: https://git.openjdk.org/jdk/pull/11915/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11915&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299836
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11915.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11915/head:pull/11915

PR: https://git.openjdk.org/jdk/pull/11915