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

[Ian Anderson via cfe-commits]

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

Cyndy made that point that `-target` and -mtargetos` aren't documented anywhere else. Put together a small paragraph describing them generally so that we don't have to have a long description for every single supported platform/variant.

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