RFR: 8292968: java.net.ContentHandler's javadoc has a broken reference

[Jaikiran Pai]

Can I please get a review of this javadoc only change which addresses https://bugs.openjdk.org/browse/JDK-8292968?

The updated javadoc now uses the `@systemProperty`, since this is the place where the semantics of this system property has been described. As per the expectations of `@systemProperty` (https://mail.openjdk.org/pipermail/core-libs-dev/2018-November/056653.html) this would then be a relevant place to use that javadoc tag. 

After this change, I've run `make docs` and verified that the generated javadoc renders properly and `java.content.handler.pkgs` shows up in the system properties listing page.

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Commit messages:
 - 8292968: java.net.ContentHandler's javadoc has a broken reference

Changes: https://git.openjdk.org/jdk/pull/10040/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10040&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292968
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10040.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10040/head:pull/10040

PR: https://git.openjdk.org/jdk/pull/10040