Add note about why bodyPiece sizing is done as it is

I think we're going to have to change this as it's quite limiting in
what you can do with osu!mania skin implementation, but for now I want
to leave a note as to why this is done, because each time I have to
trial and error check what breaks when adjusting it.

osu.Game.Rulesets.Mania/Objects/Drawables/DrawableHoldNote.cs

@@ -236,6 +236,7 @@ namespace osu.Game.Rulesets.Mania.Objects.Drawables
             };
 
             // Position and resize the body to lie half-way under the head and the tail notes.
+            // The rationale for this is account for heads/tails with corner radius.
             bodyPiece.Y = (Direction.Value == ScrollingDirection.Up ? 1 : -1) * Head.Height / 2;
             bodyPiece.Height = DrawHeight - Head.Height / 2 + Tail.Height / 2;