Add docs for the new argument in CachedCookbook#pretty_print

Since pretty_print is the interface for the stdlib PP class, we have
to accept the argument passed in if PP.pp is called with a cookbook.
This just isn't entirely clear since berkshelf uses pretty_print in a
different way, so it should be documented.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Use the more semantically correct #collect instead of #map

Signed-off-by: Evan Owen <kainosnoema@gmail.com>