add docs for propegate_from

src/repr.rs

@@ -72,7 +72,9 @@ impl Board {
         *self.get_mut(i, j) = Some(piece);
     }
 
-    // TODO! make this better and actually readable
+    /// Propegate piece captures originating from (i, j)
+    /// DO NOT USE THIS ALONE, this should be called as a part of
+    /// [`Board::place`] or [`Board::place_and_prop_unchecked`]
     fn propegate_from(&mut self, i: usize, j: usize) {
         // returns if that place is empty
         let Some(starting_color) = *self.get(i, j) else {