Add comments in response to review feedback

Clarify why `fitSideBySide` is re-run and why the `html_side_by_side`
flag is forcibly enabled for VS.

src/annotator/integrations/html.js

@@ -58,6 +58,9 @@ export class HTMLIntegration {
       const sideBySide = features.flagEnabled('html_side_by_side');
       if (sideBySide !== this._sideBySideEnabled) {
         this._sideBySideEnabled = sideBySide;
+
+        // `fitSideBySide` is normally called by Guest when the sidebar layout
+        // changes. When the feature flag changes, we need to re-run the method.
         if (this._lastLayout) {
           this.fitSideBySide(this._lastLayout);
         }

src/annotator/integrations/vitalsource.js

@@ -208,7 +208,12 @@ export class VitalSourceContentIntegration {
    */
   constructor(container = document.body) {
     const features = new FeatureFlags();
+
+    // Forcibly enable the side-by-side feature for VS books. This feature is
+    // only behind a flag for regular web pages, which are typically more
+    // complex and varied than EPUB books.
     features.update({ html_side_by_side: true });
+
     this._htmlIntegration = new HTMLIntegration({ container, features });
 
     this._listeners = new ListenerCollection();