[DOCS release] Args to {{mount}} are not documented

[mixonic]

In the docs for {{mount}} there is no mention of how arguments are managed.

• Docs link: https://www.emberjs.com/api/ember/2.15/classes/Ember.Templates.helpers/methods/mount?anchor=mount you must scroll/search the page
• https://github.com/status200/rfcs/blob/ember-engines-mount-params/text/0000-ember-engines-mount-params.md describes the behavior, but we changed the behavior after implementation began.
• 2.15 blog post Ember 2.15 Release Blog Post website#2948 has an example of usage.