[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``
   Apple's iOS operating system. The minimum deployment target is specified
   as part of the ``-target *arch*-apple-ios*version*`` command line argument.
   Alternatively, it can be specified by the ``-mtargetos=ios*version*``
   command-line argument.
 
+``maccatalyst``
+``maccatalyst_app_extension``
+  Apple's Mac Catalyst development environment for its macOS operating system.
+  The minimum deployment target is specified as part of the
+  ``-target *arch*-apple-ios*version*-macabi`` command line argument.
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cyndyishida wrote:

All of the other descriptions include `-mtargetos`, why not here? 

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