Add `merge` doc

website/source/docs/configuration/interpolation.html.md

@@ -193,6 +193,11 @@ The supported built-in functions are:
     * `map("hello", "world")`
     * `map("us-east", list("a", "b", "c"), "us-west", list("b", "c", "d"))`
 
+  * `merge(map1, map2, ...)` - Returns the union of 2 or more maps. The maps
+	are consumed in the order provided, and duplciate keys overwrite previous
+	entries.
+	* `${merge(map("a", "b"), map("c", "d"))}` returns `{"a": "b", "c": "d"}`
+
   * `md5(string)` - Returns a (conventional) hexadecimal representation of the
     MD5 hash of the given string.