Update 'doc' to support special forms and Java symbols.

The `clojure.repl/doc` format is essentially canonical, so this tests for equivalence to it. Java symbols are now supported too, though those just present a link to the javadoc.

Accompanying `cider-nrepl` PR is [#52](https://github.com/clojure-emacs/cider-nrepl/pull/52).

@bbatsov, @gtrak This also enables a handy dev trick for working with the `info` middleware: use `cider-doc` on any symbol supported by `info`, and then call `describe-text-properties` on any text in the popup buffer. The properties displayed show everything returned by the middleware.