Point the DNN migration note at Cv2.Dnn.ReadNetFromOnnx, soften engine wording

Recommend the same Cv2.Dnn facade method used elsewhere in this guide
instead of the underlying Net.ReadNetFromONNX wrapper, and clarify that
EngineType.Classic still selects the old 4.x-style engine explicitly
rather than implying it was replaced outright.

Author: shimat

docs/migration-4-to-5.md

@@ -203,7 +203,7 @@ OpenCV 5 reorganizes modules and changes some APIs. The managed surface follows
 
 - **Module reorganization** (C++ build side): `calib3d` split into `calib`/`stereo`/`geometry`/`ptcloud`; `features2d` renamed to `features`. Despite this, most C# call sites are unaffected — `Cv2.CalibrateCamera`, `Cv2.FindChessboardCorners`, `Cv2.Rodrigues`, `StereoBM`/`StereoSGBM` all keep their existing flat `Cv2`/`OpenCvSharp` locations; only the internal source file layout moved. `ml`, `gapi`, and the Haar `CascadeClassifier` / `HOGDescriptor` native modules moved into `opencv_contrib` on the **C++ build** side only — the C# `CascadeClassifier`/`HOGDescriptor` types stay in the same `OpenCvSharp` namespace and the same NuGet package, so nothing changes for callers.
 - **New `MatType` depth types**: `CV_16BF` (bfloat16), `CV_32U`, `CV_64U`, `CV_64S`, `CV_Bool`.
-- **DNN**: a new default inference engine; the Darknet and Caffe model readers were removed — convert models to ONNX and use `Net.ReadNetFromONNX`.
+- **DNN**: a new inference engine is now the default (`EngineType.Auto` tries the new engine and falls back to the classic 4.x-style engine; pass `EngineType.Classic` explicitly to force the old behavior). The Darknet and Caffe model readers were removed — convert models to ONNX and use `Cv2.Dnn.ReadNetFromOnnx`.
 - **Behavioral differences**: `Resize` with `INTER_NEAREST` now matches Pillow; `WarpAffine` / `WarpPerspective` / `Remap` interpolation produces slightly different numeric output; the legacy `HersheyFonts` text rendering changed and a new `FontFace` text API is available.
 
 _(Detailed managed signature changes will be added here per module as the port lands.)_