doc/next: add release notes for runtime and sync

Also, move the weak package heading to the end; it currently overlaps
with 1-osroot.md in the sort order.

For #68545.

Change-Id: Ia26ae511eaab7c8762e42d1593ac2197d25bbadb
Reviewed-on: https://go-review.googlesource.com/c/go/+/633599
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

doc/next/4-runtime.md

@@ -1 +1,17 @@
 ## Runtime {#runtime}
+
+<!-- go.dev/issue/54766 -->
+<!-- go.dev/cl/614795 -->
+<!-- go.dev/issue/68578 -->
+
+Several performance improvements to the runtime have decreased CPU overheads by
+2—3% on average across a suite of representative benchmarks.
+Results may vary by application.
+These improvements include a new builtin `map` implementation based on
+[Swiss Tables](https://abseil.io/about/design/swisstables), more efficient
+memory allocation of small objects, and a new runtime-internal mutex
+implementation.
+
+The new builtin `map` implementation and new runtime-internal mutex may be
+disabled by setting `GOEXPERIMENT=noswissmap` and `GOEXPERIMENT=nospinbitmutex`
+at build time respectively.

doc/next/6-stdlib/99-minor/sync/70683.md

@@ -0,0 +1,5 @@
+The implementation of [sync.Map] has been changed, improving overall performance
+and resolving some long-standing issues.
+If you encounter any problems, set `GOEXPERIMENT=nosynchashtriemap` at build
+time to switch back to the old implementation and please [file an
+issue](/issue/new).