add HtmlId for the block containing a class's description.

In the proposed new Output Spec, it is noted that ..
>The documentation derived from the documentation comment (if any) in the class or interface's source file is in a section with class description.

This is in contrast to the descriptions for a module and package, which provide ids of `module-description` and `package-description` as well as a class name.

For consistency, it would be good to provide a specific id to identify the block containing the text generated from the doc comment. Although the class is just "description", the id would be better as something like "class-description", so that it does not class with the id for any field named "description". At the same time, it might be worth renaming the class as well, again for consistency with module-description and package-description.