docs(block): add clarifying comment for overflow

This line is a bit confusing to me, so I thought I should clarify it!

See: 939da082b2

packages/mermaid/src/diagrams/block/layout.ts

@@ -272,6 +272,7 @@ function layoutBlocks(block: Block, db: BlockDB) {
       }
       let columnsFilled = child?.widthInColumns ?? 1;
       if (columns > 0) {
+        // Make sure overflowing lines do not affect later lines
         columnsFilled = Math.min(columnsFilled, columns - (columnPos % columns));
       }
       columnPos += columnsFilled;