Improve multi-tenancy docs

[mdeggies]

Document that you can add an accountStore as a parameter in application.authenticateAccount():

https://docs.stormpath.com/nodejs/jsdoc/Application.html#authenticateAccount__anchor

Show an example of this in the multi-tenancy node product guide:

https://docs.stormpath.com/nodejs/product-guide/latest/multitenancy.html#authenticating-an-account-against-an-organization

[mdeggies]

Added product guide update to this PR: stormpath/stormpath-documentation#409

[mdeggies]

Node docs update here: #582