Add documentation for {{in-element}} helper

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With the implementation of RFC#287 a new built-in helper will be added to Ember's public API. To document this helper, please add a comprehensive description of the helper to the Guides.

Please add a section on the usage of the {{in-element}} helper to the Guides, incl. at least one descriptive code example highlighting how parameters are passed and how the content of the helper's block will be rendered into the target element. This documentation addition should be made as part of the Built-In Helpers chapter.

The work on this issue should be started as soon as the public API change has been publish with a stable Ember release.