[roll third_party/pigweed] pw_presubmit: Add docs for upstream checks

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46361

third_party/pigweed Rolled-Commits: 5fecb791056c98b..99feb0bea79d3d4
Roller-URL: https://ci.chromium.org/b/8846678196484311088
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8846678196484311088
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I5bec389829ea56288c261afad52c48cde141354c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/46420
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 5fecb79..99feb0b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5fecb791056c98bae0fb1f3e145b313ab079f64a
+Subproject commit 99feb0bea79d3d4be8c5f3a70b1324fad5743912