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>