[roll third_party/pigweed] pw_rpc: Warn about pw_protobuf in docs

Updates pw_rpc/docs.rst to warn that pw_protobuf and nanopb cannot
currently exist in the same RPC server.

I hope this helps prevent others from hitting the same surprise as me.

Original-Bug: pwbug/234874320
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/127074

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: da7dc2855398dfb..a1e8808c574a645
Roller-URL: https://ci.chromium.org/b/8779547423375118305
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib34a2ddcda0423d7a0ad8f9801bf38762fe57e9f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/149811
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 da7dc28..a1e8808 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit da7dc2855398dfb6b1dc111eef8b152479952c97
+Subproject commit a1e8808c574a645eeb6800f3858435bc6043d5f7