[api] Refine description for PersistentBase::MarkActive

Bug: chromium:780749
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ifd7da70dd9adffe5de7f126539c4780799d55673
Reviewed-on: https://chromium-review.googlesource.com/763528Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49323}

include/v8.h

@@ -583,7 +583,9 @@ template <class T> class PersistentBase {
 
   /**
    * Marks the reference to this object as active. The scavenge garbage
-   * collection should not reclaim the objects marked as active.
+   * collection should not reclaim the objects marked as active, even if the
+   * object held by the handle is otherwise unreachable.
+   *
    * This bit is cleared after the each garbage collection pass.
    */
   V8_INLINE void MarkActive();