Minor changes to rustdoc html templates

This PR introduces a couple of minor changes to the rustdoc html templates:

1. The `<meta name="generator" content="rustdoc">` tag now appears in API documentation as well.
2. Adds a `rustdoc` class to the top-level `<body>` tag on all HTML pages.

The second point is a nice-to-have for those who would like to apply a user stylesheet to Rust documentation regardless of where it is hosted. Easier use of user stylesheets may alleviate much of the bikeshedding of personal taste on the rustdoc styles.