Add platform notes for building Qt Multimedia on macOS/iOS

Update src/multimedia/doc/src/qtmultimedia-building-from-source.qdoc with platform notes for iOS and macOS, including any required dependencies specific for multimedia and that are not already documented under [Qt for macOS - Building from Source | Qt 6.7|https://doc.qt.io/qt-6/macos-building.html] and [Qt for iOS - Building from Source | Qt 6.7|https://doc.qt.io/qt-6/ios-building-from-source.html]