Add docstring

cv_analysis/utils/merging.py

@@ -46,6 +46,7 @@ def merge_if_related(acc: T, pair: V) -> T:
 
 
 def bounding_rect(alpha: Rectangle, beta: Rectangle) -> Rectangle:
+    """Returns the smallest rectangle that contains both rectangles."""
     return Rectangle(
         min(alpha.x1, beta.x1),
         min(alpha.y1, beta.y1),