[roll third_party/pigweed] docs: Add style guidance for directives

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74640

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: dbb0ba614ad690b..7508836711577c8
Roller-URL: https://ci.chromium.org/b/8827740564768332577
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827740564768332577
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I957d7f0c22a3864880184b98cb2a9444c3705e33
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/74780
Bot-Commit: 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 dbb0ba6..7508836 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit dbb0ba614ad690b965b359708104fb162832f86d
+Subproject commit 7508836711577c8aade64ce0e583a67e3946bb35