[roll third_party/pigweed] pw_rpc: Update docs for channel ID remapping

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/209431
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 56b3cbf0ec8010a..12fda3bfe435881
Roller-URL: https://ci.chromium.org/b/8748383189467976241
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Change-Id: I2e2a4ef773e53b0394d95221edc3448c3662876b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/209452
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index 56b3cbf..12fda3b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 56b3cbf0ec8010a65d94d4f9b6e123f40e09ced7
+Subproject commit 12fda3bfe435881dc8f8dd9754d45cd7e1274499