roll: third_party/pigweed pw_async2: Expand docs with examples

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/234095
Original-Revision: 846bb7d3672e94c13451bec81098d3304d8395a9

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 94d2c3995c8a14..846bb7d3672e94
Roller-URL: https://ci.chromium.org/b/8735863012129066513
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iaeea4d4641b12b559c567510bf78793aa8db9926
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/237852
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 94d2c39..846bb7d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 94d2c3995c8a14995490ef7de015e933a960c2d5
+Subproject commit 846bb7d3672e94c13451bec81098d3304d8395a9