roll: third_party/pigweed pw_thread: Update docs for ThreadCore

Since switching to starting threads with pw::Function, ThreadCore is no
longer necessary. Update the docs to reflect this, and remove a few
upstream uses of pw::thread::ThreadCore.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/258455
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: f245559459ac18887fe2f63d4fb317f03583b0c3

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 462b37b0820e28..f245559459ac18
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8726436016650925889
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ifffc14cb21fdc33f00b05320faa1086ec561eb07
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/258458
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 462b37b..f245559 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 462b37b0820e284069b8e42f6e61438177b60cb7
+Subproject commit f245559459ac18887fe2f63d4fb317f03583b0c3