Add comments

lib/src/models/documents/document.dart

@@ -225,6 +225,8 @@ class Document {
           op.attributes != null ? Style.fromJson(op.attributes) : null;
 
       if (op.isInsert) {
+        // Must normalize data before inserting into the document, makes sure
+        // that any embedded objects are converted into EmbeddableObject type.
         _root.insert(offset, _normalize(op.data), style);
       } else if (op.isDelete) {
         _root.delete(offset, op.length);

lib/src/models/documents/nodes/block.dart

@@ -23,6 +23,7 @@ class Block extends Container<Line?> {
 
   @override
   Delta toDelta() {
+    // Line nodes take care of incorporating block style into their delta.
     return children
         .map((child) => child.toDelta())
         .fold(Delta(), (a, b) => a.concat(b));

lib/src/models/documents/nodes/line.dart

@@ -268,6 +268,7 @@ class Line extends Container<Leaf?> {
       unlink();
       block.insertAfter(this);
     } else {
+      /// need to split this block into two as [line] is in the middle.
       final before = block.clone() as Block;
       block.insertBefore(before);