runtime: document GODEBUG=cpu.* in runtime package

runtime: document GODEBUG=cpu.* in runtime package

So far was only documented in doc/diagnostics.

Fixes: #54183
Change-Id: Iab67a1500547f30325453c614a2f2f6621dfadb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/423275
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>