Add comment to useFollowReference

app/javascript/mastodon/hooks/useFollowReference.ts

@@ -2,6 +2,14 @@ import { useLocation } from 'react-router-dom';
 
 import type { LocationState } from '../components/router';
 
+/**
+ * Gets the current `reference` state from location state if present,
+ * otherwise returns the passed-in fallback reference.
+ *
+ * `reference` is a string that we use to anonymously & locally track
+ * where a follow is coming from, helping us understand which UI features
+ * people actually use to discover people.
+ */
 export function useFollowReference(fallbackReference: string) {
   const { state } = useLocation<LocationState>();