No documentation for typename<T>

[muglug]

Support for typename<T> was added in facebook/hhvm@3200c80 but never explicitly documented

Additionally classname<T> can be used to document a type param constraint — that's not documented either.

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• Build ID: HHVM=HHVM-4.147.0:HSL=v4.108.1:2022-03-22T21:56:11+0000:1fa47f258c6b68f8ec01899aa82fd6ffa0957109
• Page requested: /hack/built-in-types/classname
• Page requested at: Fri, 25 Mar 2022 12:56:20 +0000
• Controller: GuidePageController