Add documentation for the "typeAt" server pass.

There are some extra changes to the generated HTML when I run `make` in the doc directory - it seems the doc tools auto-detected the language type in `<pre>` code blocks and added language-appropriate styling. It does look like an improvement, but wanted to give you a heads up

Do you think I am running a different version/configuration of the doc tools, or is the `manual.html` file just stale?