docs: diff calc special behavior

Author: zxch3n

.gitignore

@@ -6,3 +6,4 @@ target
 dhat-heap.json
 .DS_Store
 node_modules/
+.idea

crates/loro-internal/src/diff_calc.rs

@@ -75,7 +75,10 @@ impl DiffCalculator {
             merged.merge(after);
             let empty_vv: VersionVector = Default::default();
             if !after.includes_vv(before) {
-                // if after is not after before, we need to calculate the diff from the beginning
+                // If after is not after before, we need to calculate the diff from the beginning
+                //
+                // This is required because of [MapDiffCalculator]. It can be removed with
+                // a better data structure. See #114.
                 before = &empty_vv;
                 after = &merged;
                 before_frontiers = None;