Add docs

packages/camera/camera_android_camerax/lib/src/android_camera_camerax.dart

@@ -227,6 +227,10 @@ class AndroidCameraCameraX extends CameraPlatform {
   /// uninitialized camera instance, this method retrieves a
   /// [ProcessCameraProvider] instance.
   ///
+  /// The specified [resolutionPreset] is the target resolution that CameraX
+  /// will attempt to select for the [UseCase]s constructed in this method
+  /// ([preview], [imageCapture], [imageAnalysis], [videoCapture]).
+  ///
   /// To return the camera ID, which is equivalent to the ID of the surface texture
   /// that a camera preview can be drawn to, a [Preview] instance is configured
   /// and bound to the [ProcessCameraProvider] instance.

packages/camera/camera_platform_interface/lib/src/types/resolution_preset.dart

@@ -5,6 +5,11 @@
 /// Affect the quality of video recording and image capture:
 ///
 /// If a preset is not available on the camera being used a preset of lower quality will be selected automatically.
+///
+/// For the CameraX implementation of the plugin on Android (`camera_android_camerax`),
+/// these are treated as a target resolution and is not guaranteed. If unavailable,
+/// a fallback resolution of the next highest quality will be attempted as
+/// mentioned above. See https://developer.android.com/media/camera/camerax/configuration#specify-resolution.
 enum ResolutionPreset {
   /// 352x288 on iOS, 240p (320x240) on Android and Web
   low,