doc: add util.types.isBig{Int,Uint}64Array()

These methods are exposed, even though the BigInt64Array and
BigUint64Array types are currently behind the --harmony-bigint
command line flag.

PR-URL: #20615
Fixes: #20602
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/util.md

@@ -1009,6 +1009,44 @@ util.types.isAsyncFunction(function foo() {});  // Returns false
 util.types.isAsyncFunction(async function foo() {});  // Returns true
 ```
 
+### util.types.isBigInt64Array(value)
+<!-- YAML
+added: v10.0.0
+-->
+
+* Returns: {boolean}
+
+Returns `true` if the value is a `BigInt64Array` instance. The
+`--harmony-bigint` command line flag is required in order to use the
+`BigInt64Array` type, but it is not required in order to use
+`isBigInt64Array()`.
+
+For example:
+
+```js
+util.types.isBigInt64Array(new BigInt64Array());   // Returns true
+util.types.isBigInt64Array(new BigUint64Array());  // Returns false
+```
+
+### util.types.isBigUint64Array(value)
+<!-- YAML
+added: v10.0.0
+-->
+
+* Returns: {boolean}
+
+Returns `true` if the value is a `BigUint64Array` instance. The
+`--harmony-bigint` command line flag is required in order to use the
+`BigUint64Array` type, but it is not required in order to use
+`isBigUint64Array()`.
+
+For example:
+
+```js
+util.types.isBigUint64Array(new BigInt64Array());   // Returns false
+util.types.isBigUint64Array(new BigUint64Array());  // Returns true
+```
+
 ### util.types.isBooleanObject(value)
 <!-- YAML
 added: v10.0.0