rustdoc: use `docblock` class more precisely

[jsha]

In Rustdoc's CSS, the docblock class generally applies to "Markdown-generated HTML that came from the user." However, right now it's also used in a few other places: the item-decl uses it, and the "All Items" page uses it. This makes maintaining the CSS more complicated, because changes that are expected to affect only one type of node actually affect many.

We should remove the docblock class from item-decl and anywhere else that isn't Markdown-generated documentation, and adjust styles as needed.