Added mutability documentation to diff

Author: taylorsw04

b+tree.ts

@@ -573,6 +573,7 @@ export default class BTree<K=any, V=any> implements ISortedMapF<K,V>, ISortedMap
    * of shared data (obtained by calling the `clone` or `with` APIs) and will avoid 
    * any iteration of shared state.
    * The handlers can cause computation to early exit by returning {break: R}.
+   * Neither of the collections should be changed during the comparison process (in your callbacks), as this method assumes they will not be mutated.
    * @param other The tree to compute a diff against.
    * @param onlyThis Callback invoked for all keys only present in `this`.
    * @param onlyOther Callback invoked for all keys only present in `other`.