[roll third_party/pigweed] pw_hdlc: Update Python RPC documents

Mention how CancellableReader and RpcClient objects must be used in
context-statements to guarantee that their teardown procedures are
invoked.
Fix RPC Python documentation section.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/174825

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 1df29eb1485fe1a..0e403d6a58aaf54
Roller-URL: https://ci.chromium.org/b/8767688749763154865
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia8a181f15e9b2a0416e72ae04634745f7f2e6c1e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/175274
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 1df29eb..0e403d6 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 1df29eb1485fe1a1cd58e2b5a7e3d8acbccfa316
+Subproject commit 0e403d6a58aaf54c811310d83e6da719e9658b9b