[DOC beta] Clarify context of a component's block.

After the changes made in #3804 (that fixed the context of template-less components) I realized that the API docs do not discuss the difference in context within the yielded block. It does follow standard Handlebars `{{yield}}` semantics, but it may not have been obvious just what this means.

The added tests were instigated by a comment from @bradleypriest [here](https://github.com/emberjs/ember.js/pull/3804#issuecomment-29889336).  I am unsure if this is the issue he was referring to, but it is nice to have the tests confirming our expectations either way.