[roll third_party/pigweed] pw_bytes: Update documentation

Add doxygen directives for byte_builder.h.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/188462

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: bc7e53c42364578..2b6959a8ecd271d
Roller-URL: https://ci.chromium.org/b/8758168721986490417
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I166550c38174dd47590f11b43612bad9cb2b2b1a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/188737
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 bc7e53c..2b6959a 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit bc7e53c423645785cec1346be650d5c95fadcdaa
+Subproject commit 2b6959a8ecd271d51c302711fb7e4ff7ea41628e