RFR: 8288524: Allow @systemProperty to appear in overview documentation

[Jonathan Gibbons]

On Tue, 21 Jun 2022 19:23:30 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

> 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

Marked as reviewed by jjg (Reviewer).

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