The strange thing is that you have documentation in minibuffer with this configuration. Maybe your distribution also loads it from a different location.
Please try this config:
(setq inferior-lisp-program "/usr/bin/sbcl")
(add-to-list 'load-path "~/.emacs.d/slime-2012-04-14/")
(require 'slime)
(require 'slime-autoloads)
(slime-setup '(slime-autodoc))
It tells Emacs to load and use slime-autodoc
module that displays documentation and in minibuffer and highlights it as you'd like it to be.
Maybe you'd also like to update to a more recent SLIME version (the current one in ELPA is 20130402).