Javadoc on hover

When we hover variables, types, methods, etc, we're only showing a brief signature. We could add the javadoc of those symbols here as well, much like the Java language server does.

Start by locating the language server's hover handling and the code that formats the current symbol signature. Compare the intended behavior with the Java language server, then verify that hovering variables, types, methods, and other symbols includes their available Javadoc alongside the signature.

Written by the indexing model from the issue text.