Developing clojure with Emacs Cider is great. To be able to view summaries for clojure functions and special forms, type this in you Cider REPL:
(use 'clojure.repl)
Afterwards, you can do stuff like this:
my.repl> (doc conj)
-------------------------
clojure.core/conj
([coll x] [coll x & xs])
conj[oin]. Returns a new collection with the xs
'added'. (conj nil item) returns (item). The 'addition' may
happen at different 'places' depending on the concrete type.
nil