update docs for $reduce function

Author: andrew-coleman

docs/higher-order-functions.md

@@ -47,7 +47,7 @@ The function that is supplied as the second parameter must have the following si
 
 Each value in the input array is passed in as the first parameter in the supplied function.  The index (position) of that value in the input array is passed in as the second parameter, if specified.  The whole input array is passed in as the third parameter, if specified.
 
-__Example__  
+__Example__
 The following expression returns all the products whose price is higher than average:
 ```
 $filter(Account.Order.Product, function($v, $i, $a) {
@@ -66,7 +66,7 @@ The function that is supplied as the second parameter must have the following si
 
 Each value in the input array is passed in as the first parameter in the supplied function.  The index (position) of that value in the input array is passed in as the second parameter, if specified.  The whole input array is passed in as the third parameter, if specified.
 
-__Example__  
+__Example__
 The following expression the product in the order whose SKU is `"0406654608"`:
 ```
 $single(Account.Order.Product, function($v, $i, $a) {
@@ -79,7 +79,9 @@ __Signature:__ `$reduce(array, function [, init])`
 
 Returns an aggregated value derived from applying the `function` parameter successively to each value in `array` in combination with the result of the previous application of the function.
 
-The function must accept two arguments, and behaves like an infix operator between each value within the `array`.
+The `function` must accept at least two arguments, and behaves like an infix operator between each value within the `array`.  The signature of this supplied function must be of the form:
+
+`myfunc($accumulator, $value[, $index[, $array]])`
 
 __Example__