runtime: document pointer write atomicity for memclrNoHeapPointers

memclrNoHeapPointers is the underlying implementation of
typedmemclr and memclrHasPointers, so it still needs to write
pointer-aligned words atomically. Document this requirement.

Updates #41428.

Change-Id: Ice00dee5de7a96a50e51ff019fcef069e8a8406a
Reviewed-on: https://go-review.googlesource.com/c/go/+/287692


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>