[roll third_party/pigweed] pw_chrono: Docs update regarding conversion of std::chrono::duration's

Make it more clear that when using the std::chrono helper durations,
conversion is required. Add note mentioning for_at_least as an option
when compatibility with SystemClock::duration is wanted.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/75200

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: b7dc33f8af47ec2..af2fb1f0e4d214a
Roller-URL: https://ci.chromium.org/b/8827293774439900881
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827293774439900881
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7e1765efa4bf5cd2443528ecd37bf184e425f1e6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/75541
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 b7dc33f..af2fb1f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit b7dc33f8af47ec22d4777f3d89a175016a8c162d
+Subproject commit af2fb1f0e4d214a91de3a3fcdd17ac61053e72cb