roll: third_party/pigweed pw_thread: Comment / docs updates

- Discourage use of ThreadCore in new code, since threads are now
  constructed from pw::Function<void()>.
- Remove references to default-constructed Options, since that is not
  supported.
- Formatting updates.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/236795
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 25c8b842ecfa6d530b785d61c1fab105b352bcdd

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: f1f654a15a3adc..25c8b842ecfa6d
Roller-URL: https://ci.chromium.org/b/8736413999913380721
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I98e4d7be1163c21405134e00893311cc9f120fc3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/236855
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index f1f654a..25c8b84 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f1f654a15a3adce476c2d68643eee56f3c225dd4
+Subproject commit 25c8b842ecfa6d530b785d61c1fab105b352bcdd