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

[Cyndy Ishida via cfe-commits]

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@@ -1592,6 +1592,12 @@ succeeds but Clang emits a warning specifying that the function is deprecated.
 Finally, if Clang is instructed to compile code for macOS 10.7, the call
 fails because ``f()`` is no longer available.
 
+Clang is instructed to compile code for a particular OS version using the
+``-target`` or ``-mtargetos`` flags. For example, macOS 10.7 would be
----------------
cyndyishida wrote:

```suggestion
``-target`` or ``-mtargetos`` command line arguments. For example, macOS 10.7 would be
```

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