[roll third_party/pigweed] pw_rust: Add combined Rust doc support

Relies on protoype of rules_rust support for generating docs for
multiple crates at once.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/157632

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: cd1bc6cfd4146c3..43dfeb82765d48c
Roller-URL: https://ci.chromium.org/b/8775013713623505329
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id766914178204d60a04592072687c1837dab9acd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/157770
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 cd1bc6c..43dfeb8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit cd1bc6cfd4146c31db20558ea3c8c668dad505c6
+Subproject commit 43dfeb82765d48c963a5393d8af8201e830245b0