doc: add release notes for hot block alignment

This is for the changes in CL 577935.

Change-Id: I8f70dd36c89aaede11a37744c0f279439e01eb13
Reviewed-on: https://go-review.googlesource.com/c/go/+/586895
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>

doc/next/5-toolchain.md

@@ -9,6 +9,13 @@ The compiler in Go 1.23 can now overlap the stack frame slots of local variables
 accessed in disjoint regions of a function, which reduces stack usage
 for Go applications.
 
+<!-- https://go.dev/cl/577935 -->
+For 386 and amd64, the compiler will use information from PGO to align certain
+hot blocks in loops.  This improves performance an additional 1-1.5% at
+a cost of an additional 0.1% text and binary size.  This is currently only implemented
+on 386 and amd64 because it has not shown an improvement on other platforms.
+Hot block alignment can be disabled with `-gcflags=[<packages>=]-d=alignhot=0`
+
 ## Assembler {#assembler}
 
 ## Linker {#linker}