[roll third_party/pigweed] docs: add OS abstraction layers doc

Adds a new top level doc on Pigweed's OS abstraction layers.
While doing so, also adds some minor updates to the relevant modules.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44920

third_party/pigweed Rolled-Commits: 5dac14a79cad82d..cc756c84e836013
Roller-URL: https://ci.chromium.org/b/8847409107714259520
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8847409107714259520
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie9a02558afb400a59ff5695ed9dbccd84995a023
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/44945
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 5dac14a..cc756c8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5dac14a79cad82d2beeff6bdd7cbbfcfe317b9d0
+Subproject commit cc756c84e8360136cd40ef5898df2cca61b799cc