doc: mention prototype check in deepStrictEqual()

The docs for assert.deepStrictEqual() do not currently mention
that prototypes are compared for objects. This commit adds that
information to the documentation.

Fixes: https://github.com/nodejs/node/issues/5365
PR-URL: https://github.com/nodejs/node/pull/5367
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: Benjamin Gruenbaum <inglor@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/assert.markdown

@@ -89,8 +89,9 @@ parameter is undefined, a default error message is assigned.
 
 ## assert.deepStrictEqual(actual, expected[, message])
 
-Generally identical to [`assert.deepEqual()`][] with the exception that primitive
-values are compared using the strict equality operator ( `===` ).
+Generally identical to `assert.deepEqual()` with two exceptions. First,
+primitive values are compared using the strict equality operator ( `===` ).
+Second, object comparisons include a strict equality check of their prototypes.
 
 ```js
 const assert = require('assert');