Added a description for Classic subsection for Element Modifiers

[ijlee2]

Description

This PR addresses and can close #45.

I opted for adding a paragraph (i.e. a translation key) that explains that @ember/render-modifiers can still be used in classic apps as long as the Ember version is 2.18 or higher.

Notes

The alternative approach that I had thought of was to update the <GuideSection::Subsection> component template to conditionally render a does-not-apply message. I thought adding the conditional logic,

{{#if (or @subsection.classicFiles @subsection.classicDescriptionKey)}}
  {{!-- Show classicFiles or classicDescriptionKey --}}
{{else}}
  {{!-- Show does-not-apply message --}}
{{/lf}}

one for Classic and yet another for Octane, might cause the template to be messy now or later.

[amyrlam]

Seems like a good improvement!