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 Int32: sig .. end
   This module provides operations on the type int32
   of signed 32-bit integers.  Unlike the built-in int type,
   the type int32 is guaranteed to be exactly 32-bit wide on all
   platforms.  All arithmetic operations over int32 are taken
   modulo 232.
   Performance notice: values of type int32 occupy more memory
   space than values of type int, and arithmetic operations on
   int32 are generally slower than those on int.  Use int32
   only when the application requires exact 32-bit arithmetic.
val zero : int32
val one : int32
val minus_one : int32
val neg : int32 -> int32
val add : int32 -> int32 -> int32
val sub : int32 -> int32 -> int32
val mul : int32 -> int32 -> int32
val div : int32 -> int32 -> int32
Division_by_zero if the second
   argument is zero.  This division rounds the real quotient of
   its arguments towards zero, as specified for Pervasives.(/).val rem : int32 -> int32 -> int32
y is not zero, the result
   of Int32.rem x y satisfies the following property:
   x = Int32.add (Int32.mul (Int32.div x y) y) (Int32.rem x y).
   If y = 0, Int32.rem x y raises Division_by_zero.val succ : int32 -> int32
Int32.succ x is Int32.add x Int32.one.val pred : int32 -> int32
Int32.pred x is Int32.sub x Int32.one.val abs : int32 -> int32
val max_int : int32
val min_int : int32
val logand : int32 -> int32 -> int32
val logor : int32 -> int32 -> int32
val logxor : int32 -> int32 -> int32
val lognot : int32 -> int32
val shift_left : int32 -> int -> int32
Int32.shift_left x y shifts x to the left by y bits.
   The result is unspecified if y < 0 or y >= 32.val shift_right : int32 -> int -> int32
Int32.shift_right x y shifts x to the right by y bits.
   This is an arithmetic shift: the sign bit of x is replicated
   and inserted in the vacated bits.
   The result is unspecified if y < 0 or y >= 32.val shift_right_logical : int32 -> int -> int32
Int32.shift_right_logical x y shifts x to the right by y bits.
   This is a logical shift: zeroes are inserted in the vacated bits
   regardless of the sign of x.
   The result is unspecified if y < 0 or y >= 32.val of_int : int -> int32
int) to a 32-bit integer
    (type int32).val to_int : int32 -> int
int32) to an
   integer (type int).  On 32-bit platforms, the 32-bit integer
   is taken modulo 231, i.e. the high-order bit is lost
   during the conversion.  On 64-bit platforms, the conversion
   is exact.val of_float : float -> int32
Int32.min_int, Int32.max_int].val to_float : int32 -> float
val of_string : string -> int32
0x, 0o or 0b
   respectively.
   Raise Failure "int_of_string" if the given string is not
   a valid representation of an integer, or if the integer represented
   exceeds the range of integers representable in type int32.val to_string : int32 -> string
val bits_of_float : float -> int32
val float_of_bits : int32 -> float
int32.type t = int32 
val compare : t -> t -> int
Pervasives.compare.  Along with the type t, this function compare
    allows the module Int32 to be passed as argument to the functors
    Set.Make and Map.Make.val format : string -> int32 -> string