[roll third_party/pigweed] pw_channel: Docs fix

- Specify the namespace in the API docs, since it is not included in the
  Doxygen group.
- Remove explicit values from Whence because the values are not
  significant.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/190624

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 674294db69c284a..b31d94d6d972466
Roller-URL: https://ci.chromium.org/b/8756735253191263121
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2c25803754cdeb00c08c35be77cc0f76a93d42c1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/190690
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 674294d..b31d94d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 674294db69c284a29c2aa86929246d19c8d3c770
+Subproject commit b31d94d6d97246655c010cf9043692f511a8d7cd