Update assert.markdown

The current wording "This module is used for writing unit tests for your applications, you can access it with require('assert')." implies that this module should only be used in development while unit testing.

The article "Error Handling in Node.js" by Joyent (https://www.joyent.com/developers/node/design/errors) uses the assert module in an efficient way to validate required function arguments.

PR-URL: https://github.com/joyent/node/pull/25811


Reviewed-By: James M Snell <jasnell@gmail.com>