[roll pigweed] docs: Minor additions for preprocessor macro style

- Reference the preprocessor macro usage guidelines from the macro
  naming guidelines section.
- Link the GCC docs about swallowing the semicolon in macros.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98440

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 6c5df358e638a6f..50b6f3784a24bf8
Roller-URL: https://ci.chromium.org/b/8810066856484224385
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8810066856484224385
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0caabc08441006a0002ebabebf5decaeffb83498
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/100341
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/pigweed b/pigweed
index 6c5df35..50b6f37 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 6c5df358e638a6f6677c8f000170266c836f674d
+Subproject commit 50b6f3784a24bf89948de1eb5c3a49d6288e5493