[roll third_party/pigweed] pw_allocator: Doxygenify Block:FromUsableSpace()

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/151210

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d4d9e3cbd4f6c9f..a4adf3b093a202f
Roller-URL: https://ci.chromium.org/b/8778470813464798081
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I99ce94d5a15be4992a408f613707b95780ed039c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/151516
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 d4d9e3c..a4adf3b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d4d9e3cbd4f6c9f32f445a69b23d62f3f78b1fb7
+Subproject commit a4adf3b093a202f99276edde598987bc992aeb14