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

[Egor Zhdan 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``.
+  This allows Swift to correctly manage the retain and release operations at the
+  language boundary.
+
+  The possible values are:
+
+  - ``retained`` — Indicates that the function or method returns a +1 reference.
+    Swift will take ownership and is responsible for releasing it.
+    Equivalent to ``SWIFT_RETURNS_RETAINED``.
+  - ``unretained`` — Indicates that the function or method returns a +0 reference.
+    The caller must ensure the returned object remains alive.
+    Equivalent to ``SWIFT_RETURNS_UNRETAINED``.
+
+  This attribute can be applied to:
+
+  - **C++ functions**
+  - **C++ instance or static methods**
+  - **Objective-C / Objective-C++ methods or functions that return a C++ reference type**
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egorzhdan wrote:

Minor: I'd say let's not highlight this in bold. Since we don't use bold highlighting elsewhere in this document, this would attract too much attention.

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