[roll third_party/pigweed] pw_sync: expansion of the sync module doc

Expands the synchronization doc, adding current and planned
supported environments for primitives and what primitives are on
our short term roadmap.

Also adds a warning to Mutex that we will likely split it into
a Mutex and separate TimedMutex.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39908

third_party/pigweed Rolled-Commits: dfdce5256eba595..3b9eca49b06c923
Roller-URL: https://ci.chromium.org/b/8851017965834473632
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8851017965834473632
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iad2097d5616a5dd6baa935fb3e5f53ab5cec7c7f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/39915
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index dfdce52..3b9eca4 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit dfdce5256eba595b3cb085561e03d41460e62260
+Subproject commit 3b9eca49b06c923d8dbee72d092572aa66844e0c