Provide self-contained examples for =, as and super on the constraints page

[Wilfred]

The examples on this page are prose-heavy and hard to understand.

Please don't change anything below this point.

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• Build ID: HHVM=HHVM-4.154.0:HSL=v4.108.1:2022-05-03T20:54:00+0000:1fa47f258c6b68f8ec01899aa82fd6ffa0957109
• Page requested: /hack/generics/type-constraints
• Page requested at: Fri, 13 May 2022 19:59:45 +0000
• Controller: GuidePageController