[roll third_party/pigweed] pw_rpc: Documentation updates

- Expand the RPC call lifecycle to discuss call types and events.
- Describe the public C++ call classes.
- Describe the limitations of and restrictions on C++ callbacks. These
  restrictions will be introduced by upcoming CLs addressing b/234876851
  and b/262774186.
- Update headings to match the style guide.
- Update references to the old client API.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/126676

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4fdddace5d9772d..b7b0a19a3aa4b81
Roller-URL: https://ci.chromium.org/b/8791663454080362849
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I369d718fe93d6a43b9a32c5c5b3857e1f499445a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/126699
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 4fdddac..b7b0a19 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4fdddace5d9772dcacb75d5d8409a483ac90391d
+Subproject commit b7b0a19a3aa4b8143b3bf1308bf01dc56bc1bcdc