RFR: JDK-8247608: Javadoc: CSS margin is not applied consistently

[Hannes Wallnöfer]

This simple stylesheet change adds a 10px right side border to the `<dd>` element containing the contents of block tags. I'm attaching a screenshot with the fixed spacing, see JBS attachments for reproducer source code and before-screenshots.

<img width="713" alt="border" src="https://user-images.githubusercontent.com/15975/119998289-db671200-bfd0-11eb-95f7-a89ffe10cff4.png">

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Commit messages:
 - JDK-8247608: Javadoc: CSS margin is not applied consistently

Changes: https://git.openjdk.java.net/jdk/pull/4250/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4250&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247608
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4250.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4250/head:pull/4250

PR: https://git.openjdk.java.net/jdk/pull/4250