[clang] [NFC] [API Notes] [Documentation] Add documentation for SwiftReturnOwnership (PR #143545)

[Gábor Horváth via cfe-commits]

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@@ -195,6 +195,52 @@ declaration kind), all of which are optional:
       SwiftReleaseOp: immortal
       SwiftRetainOp: immortal
 
+:SwiftReturnOwnership:
+
+  Specifies the ownership convention of a function or method returning a C++ type
+  that has been imported as a Swift reference type using ``SwiftImportAs: reference``.
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Xazax-hun wrote:

> using ``SwiftImportAs: reference``

Technically, is it possible that the type is annotated via attributes and not API notes? If that is the case, I think we can just drop the "using" part as it is not necessarily true. 

https://github.com/llvm/llvm-project/pull/143545