doc: mention stack allocation of slices

Very similar to last release's note.

Change-Id: Ie7afe21d98cee1c9718e53b20e8af8ee18504bb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/725921
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>

Author: randall77

doc/next/5-toolchain.md

@@ -1,5 +1,14 @@
 ## Compiler {#compiler}
 
+<!-- CLs 707755, 722440 -->
+
+The compiler can now allocate the backing store for slices on the stack in more
+situations, which improves performance. If this change is causing trouble, the
+[bisect tool](https://pkg.go.dev/golang.org/x/tools/cmd/bisect) can be used to
+find the allocation causing trouble using the `-compile=variablemake` flag. All
+such new stack allocations can also be turned off using
+`-gcflags=all=-d=variablemakehash=n`.
+
 ## Assembler {#assembler}
 
 ## Linker {#linker}