docs: note desktop audio limitation on macOS (#17815)

docs/api/desktop-capturer.md

@@ -92,3 +92,9 @@ objects, each `DesktopCapturerSource` represents a screen or an individual windo
 captured.
 
 [`navigator.mediaDevices.getUserMedia`]: https://developer.mozilla.org/en/docs/Web/API/MediaDevices/getUserMedia
+
+### Caveats
+
+`navigator.mediaDevices.getUserMedia` does not work on macOS for audio capture due to a fundamental limitation whereby apps that want to access the system's audio require a [signed kernel extension](https://developer.apple.com/library/archive/documentation/Security/Conceptual/System_Integrity_Protection_Guide/KernelExtensions/KernelExtensions.html). Chromium, and by extension Electron, does not provide this.
+
+It is possible to circumvent this limitation by capturing system audio with another macOS app like Soundflower and passing it through a virtual audio input device. This virtual device can then be queried with `navigator.mediaDevices.getUserMedia`.