docs(debounce): clarify that cancel() drops, never flushes, the trailing call

Markus on PR #629 — the cancel-not-flush contract is what the
PersonMentionEditor onDestroy path relies on. Spell it out so future
callers can rely on the same guarantee.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/utils/debounce.ts

@@ -1,9 +1,9 @@
 /**
  * Returns a debounced version of fn that delays invocation until after
  * `delay` ms have elapsed since the last call. The returned function
- * exposes a `cancel()` method that clears any pending invocation —
- * essential when the host context (a destroyed component, an unmounted
- * editor) shouldn't fire the trailing call.
+ * exposes a `cancel()` method that DROPS (does not flush) the pending
+ * trailing invocation — essential when the host context (a destroyed
+ * component, an unmounted editor) shouldn't fire the trailing call.
  */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function debounce<T extends (...args: any[]) => void>(