which protocol defines conj in clojure?

lets say I wrote a function:

(defn foo [to x] (conj to x))

and would like to document it by stating that to must implement some protocol (as in the structure/type to must support the call conj). Is there a website or database that has this information? Obviously I would like to generalise this question to "where can I find a complete reference for all clojure protocols?"

As a clear and concrete example using Sam Estep's suggestion it would look like:

(defn invert-many-to-one
  "returns a one-to-many mapping where vals are collections of type `(constructor-fn)`,
   (defaults to `hash-set`). Note that `constructor-fn` is a function of 0 args.
  `insert-fn` function can be passed. if only `constructor-fn` is passed
  then `insert-fn` defaults to `conj` and `(constructor-fn)` must be an instance
  of `clojure.lang.IPersistentCollection`"
  ([m] (invert-many-to-one hash-set conj m))
  ([constructor-fn m] {:pre [(instance? clojure.lang.IPersistentCollection (constructor-fn))]}
   (invert-many-to-one constructor-fn conj m))
  ([constructor-fn insert-fn m]
   (persistent!
    (reduce (fn [m [k v]]
              (assoc! m v (insert-fn (clojure.core/get m v (constructor-fn)) k)))
            (transient {}) m))))