Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
module Make: functor (Ord : OrderedType) -> S  with type key = Ord.t| Parameters: | 
 | 
type key 
type +'a t 
key to type 'a.val empty : 'a t
val is_empty : 'a t -> bool
val mem : key -> 'a t -> bool
mem x m returns true if m contains a binding for x,
       and false otherwise.val add : key -> 'a -> 'a t -> 'a t
add x y m returns a map containing the same bindings as
       m, plus a binding of x to y. If x was already bound
       in m, its previous binding disappears.val singleton : key -> 'a -> 'a t
singleton x y returns the one-element map that contains a binding y
        for x.val remove : key -> 'a t -> 'a t
remove x m returns a map containing the same bindings as
       m, except for x which is unbound in the returned map.val merge : (key -> 'a option -> 'b option -> 'c option) -> 'a t -> 'b t -> 'c t
merge f m1 m2 computes a map whose keys is a subset of keys of m1
        and of m2. The presence of each such binding, and the corresponding
        value, is determined with the function f.val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> intval equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> boolequal cmp m1 m2 tests whether the maps m1 and m2 are
       equal, that is, contain equal keys and associate them with
       equal data.  cmp is the equality predicate used to compare
       the data associated with the keys.val iter : (key -> 'a -> unit) -> 'a t -> unit
iter f m applies f to all bindings in map m.
       f receives the key as first argument, and the associated value
       as second argument.  The bindings are passed to f in increasing
       order with respect to the ordering over the type of the keys.val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
fold f m a computes (f kN dN ... (f k1 d1 a)...),
       where k1 ... kN are the keys of all bindings in m
       (in increasing order), and d1 ... dN are the associated data.val for_all : (key -> 'a -> bool) -> 'a t -> bool
for_all p m checks if all the bindings of the map
        satisfy the predicate p.val exists : (key -> 'a -> bool) -> 'a t -> bool
exists p m checks if at least one binding of the map
        satisfy the predicate p.val filter : (key -> 'a -> bool) -> 'a t -> 'a t
filter p m returns the map with all the bindings in m
        that satisfy predicate p.val partition : (key -> 'a -> bool) -> 'a t -> 'a t * 'a t
partition p m returns a pair of maps (m1, m2), where
        m1 contains all the bindings of s that satisfy the
        predicate p, and m2 is the map with all the bindings of
        s that do not satisfy p.val cardinal : 'a t -> int
val bindings : 'a t -> (key * 'a) list
Ord.compare, where Ord is the argument
       given to Map.Make.val min_binding : 'a t -> key * 'a
Ord.compare ordering), or raise
       Not_found if the map is empty.val max_binding : 'a t -> key * 'a
val choose : 'a t -> key * 'a
Not_found if
       the map is empty. Which binding is chosen is unspecified,
       but equal bindings will be chosen for equal maps.val split : key -> 'a t -> 'a t * 'a option * 'a t
split x m returns a triple (l, data, r), where
          l is the map with all the bindings of m whose key
        is strictly less than x;
          r is the map with all the bindings of m whose key
        is strictly greater than x;
          data is None if m contains no binding for x,
          or Some v if m binds v to x.val find : key -> 'a t -> 'a
find x m returns the current binding of x in m,
       or raises Not_found if no such binding exists.val map : ('a -> 'b) -> 'a t -> 'b tmap f m returns a map with same domain as m, where the
       associated value a of all bindings of m has been
       replaced by the result of the application of f to a.
       The bindings are passed to f in increasing order
       with respect to the ordering over the type of the keys.val mapi : (key -> 'a -> 'b) -> 'a t -> 'b t
Map.S.map, but the function receives as arguments both the
       key and the associated value for each binding of the map.