Add a couple of notes in comments

These address a couple of issues raised in code review.

src/sidebar/helpers/annotation-metadata.js

@@ -322,6 +322,8 @@ export function location(annotation) {
   let cfi;
   let position;
 
+  // nb. We ignore the possibility of an annotation having multiple targets here.
+  // h and the client only support one.
   for (const selector of targets[0]?.selector ?? []) {
     if (selector.type === 'TextPositionSelector') {
       position = selector.start;

src/sidebar/helpers/thread-sorters.js

@@ -96,6 +96,7 @@ export const sorters = {
     if (aLocation.cfi && bLocation.cfi) {
       const cfiResult = compareCFIs(aLocation.cfi, bLocation.cfi);
       if (cfiResult !== 0) {
+        // Annotations are in different chapters.
         return Math.sign(cfiResult);
       }
     } else if (aLocation.cfi) {