[roll third_party/pigweed] docs: Add details about doc requirements

Original-Bug: 253
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39480

third_party/pigweed Rolled-Commits: 48c43dd8976cb1f..895423426369958
Roller-URL: https://ci.chromium.org/b/8851392779735792288
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8851392779735792288
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib11ce3c40dcb53547d9d30f261be5a9dfc00c181
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/39503
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 48c43dd..8954234 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 48c43dd8976cb1fc7e11c30c9ffe9c5a8706150c
+Subproject commit 89542342636995801953b8eadaab3e2ee9a221f7