Document options for render in API [ci skip]

Creates parity with the [guides][g] by documenting the [variants][v]
option.

[g]: https://guides.rubyonrails.org/v8.0/layouts_and_rendering.html#options-for-render
[v]: https://guides.rubyonrails.org/v8.0/layouts_and_rendering.html#the-variants-option

Author: stevepolitodesign

actionpack/lib/action_controller/metal/rendering.rb

@@ -160,6 +160,12 @@ def inherited(klass)
     #         render "posts/new", status: :unprocessable_entity
     #         # => renders app/views/posts/new.html.erb with HTTP status code 422
     #
+    # `:variants`
+    # :  This tells Rails to look for the first template matching any of the variations.
+    #
+    #         render "posts/index", variants: [:mobile]
+    #         # => renders app/views/posts/index.html+mobile.erb
+    #
     #--
     # Check for double render errors and set the content_type after rendering.
     def render(*args)