roll: pigweed pw_rpc2: Add protocol doc

Documents the protocol, handshake, packet types, and error codes.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/485249
Original-Revision: ef912bbec6b63befae0dcba2fb91b88e504be8e2

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 14d770dd520991..ef912bbec6b63b
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8669653442949758689
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9951d8956a5f27825bf84c9ceb2ca642c7db4bb2
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/486229

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index b507241..b23e3a7 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -41,8 +41,8 @@
 git_override(
     module_name = "pigweed",
     # ROLL: Warning: this is automatically updated by
-    # ROLL: https://cr-buildbucket.appspot.com/build/8669664286039085377
-    commit = "14d770dd520991d74804f1b441645224cacc8cdb",
+    # ROLL: https://cr-buildbucket.appspot.com/build/8669653442949758689
+    commit = "ef912bbec6b63befae0dcba2fb91b88e504be8e2",
     remote = "https://pigweed.googlesource.com/pigweed/pigweed",
 )