[clang] [docs] Document the missing availability platforms and environments (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:

I do think it's still useful to define what those platforms' values are, for the most part, it is an easy search away though. 

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