RFR: 8288524: Allow @systemProperty to appear anywhere

[Pavel Rappo]

Please review this trivial change. Note that the meat of the change is well-tested by the following existing tests:

-   test/langtools/jdk/javadoc/doclet/testSystemPropertyPage
-   test/langtools/jdk/javadoc/doclet/testIndexInDocFiles

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

Commit messages:
 - Change the taglet and a test

Changes: https://git.openjdk.org/jdk19/pull/56/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=56&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288524
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk19/pull/56.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/56/head:pull/56

PR: https://git.openjdk.org/jdk19/pull/56