[roll third_party/pigweed] pw_chrono_rp2040: Minor tweaks to documentation and test

Surfaces more information about when to use this pw_chrono backend. Also
updates the test to poll now() rather than doing arbitrary work to
hopefully be less likely to be prone to flake (not that any flakes were
observed with the old test design).

Original-Fixes: b/303297807
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/183730

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e982c93aba2cc25..e8de610d576dd4a
Roller-URL: https://ci.chromium.org/b/8755911792413152401
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia9c1fa995d1008b5ec682c89f119f6a0ced5764f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/192788
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 e982c93..e8de610 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit e982c93aba2cc259e65cc3e518ecf3230e5d2d9f
+Subproject commit e8de610d576dd4a92564fbbc0bc2ff9a9d228627