roll: third_party/pigweed: pw_sync: Document that CV should not be used

Original-Bug: b/294395229

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/162771
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 707473a3602a6ab..e15bd2067e7a244
Roller-URL: https://ci.chromium.org/b/8742585682393481057
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I81b1832a06708456e7d056cf239bab51311b9ef9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/222853
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 707473a..e15bd20 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 707473a3602a6abedeec3fe612aa0e02d043f586
+Subproject commit e15bd2067e7a244ff3d8d2c33468407865367af3