Add translations for image annotations in frontend, explanation for

editing one in musdb

backend/pl/image_incha.json

@@ -49,7 +49,8 @@
         "annotate_image": "Annotate image",
         "added_annotation": "Added annotation",
         "updated_annotation": "Updated annotation",
-        "deleted_annotation": "Deleted annotation"
+        "deleted_annotation": "Deleted annotation",
+        "annotation_explica": "To annotate an image section, click into the image at the top left of the area you want to annotate. Then, move the cursor to the bottom right of the area to be annotated and release the click. A sidebar will open, offering you to enter your annotation."
     },
     "incha_image": {
         "front": "Widok od przodu",