roll: third_party/pigweed pw_kvs: Rework documentation structure & content

Rework the KVS docs to better explain how to use it, and offer a more
detailed explanation of the disk format and its design.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/325501
Original-Revision: 5787baca2f46f5e9831ffdd7d40ace996cd34701

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 5bdabe588948fa..5787baca2f46f5
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8702529921791048705
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If9ffd822fbc7dbdddb6cbea26879d2872b33aff8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/326592
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 5bdabe5..5787bac 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5bdabe588948fa10c613ae06acce097339a824b4
+Subproject commit 5787baca2f46f5e9831ffdd7d40ace996cd34701