RFR: JDK-8223357: No marker comment for a package in package-summary.html

[Hannes Wallnöfer]

Please review yet another simple fix to add a marker comment to the start of the package description in `package-summary.html` files.

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Commit messages:
 - JDK-8223357: No marker comment for a package in package-summary.html

Changes: https://git.openjdk.java.net/jdk/pull/5947/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5947&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8223357
  Stats: 11 lines in 3 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5947.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5947/head:pull/5947

PR: https://git.openjdk.java.net/jdk/pull/5947