[roll third_party/pigweed] docs: Add example of conditionally enabling code based on module config

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/187171

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d3c6af1625f3e77..51936c939a01ab9
Roller-URL: https://ci.chromium.org/b/8759245659082957409
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I35b31498797523eea111c47402ad11291ed89b60
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/187391
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index d3c6af1..51936c9 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d3c6af1625f3e772587ad271eb25ca9b594b3f96
+Subproject commit 51936c939a01ab97a33929711c4bc1e8e5d40e6e