RFR: JDK-8263300: add HtmlId for the block containing a class's description.

[Hannes Wallnöfer]

This is a relatively simple change to add an `id="class-description"` to the description section of type pages. It also changes the CSS class for this section from `description` to `class-description` for consistency with package and module pages. 

My initial plan was to move the beginning of the section to after the type signature, as this is how it is done for package and module pages. However, I'm not sure there's any benefit to this that would be worth the change, so I decided to keep it simple.

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Commit messages:
 - JDK-8263300: add HtmlId for the block containing a class's description.

Changes: https://git.openjdk.java.net/jdk/pull/2971/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2971&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263300
  Stats: 11 lines in 7 files changed: 2 ins; 1 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2971.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2971/head:pull/2971

PR: https://git.openjdk.java.net/jdk/pull/2971