[clang] [docs] Document the missing Apple availability platforms (PR #104653)

[Cyndy Ishida via cfe-commits]

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@@ -1637,31 +1637,42 @@ specifies availability for the current target platform, the availability
 attributes are ignored. Supported platforms are:
 
 ``ios``
+``ios_app_extension``
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cyndyishida wrote:

The description only describes the platform. Can you also include an explanation for app_extension? 

https://github.com/llvm/llvm-project/pull/104653