[roll] Roll fuchsia [docs] Add doc describing kernel thread signaling

Add some internal kernel-developer-centric documentation for thread
signaling.

This doc was previously drafted+reviewed in Google Docs form.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/581875
Original-Revision: ba4b58eae1744c1c5a3301d4789c38de9c79515f
GitOrigin-RevId: 19892a568f8544b73fde01cadec644912eb56ba6
Change-Id: I307d5dfd68d774780e388199e0650467a483c3f0
diff --git a/stem b/stem
index 3901b1e..4014173 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="52a612e765e995497bc611da973b909f0bbca241"/>
+             revision="ba4b58eae1744c1c5a3301d4789c38de9c79515f"/>
   </projects>
   <hooks>
     <hook name="install-environment"