runtime: clarify which work needs spinning coordination

The overview comments discuss readying goroutines, which is the most
common source of work, but timers and idle-priority GC work also require
the same synchronization w.r.t. spinning Ms.

This CL should have no functional changes.

For #43997
Updates #44313

Change-Id: I7910a7f93764dde07c3ed63666277eb832bf8299
Reviewed-on: https://go-review.googlesource.com/c/go/+/307912


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>