cmd/compile: expand comment on cgo_unsafe_args in inlinine

Change-Id: Ie8fd7839806b4a6bcfac5ac1f984db7ab282c3b5
Reviewed-on: https://go-review.googlesource.com/41150
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Lazar <lazard@golang.org>

src/cmd/compile/internal/gc/inl.go

@@ -111,7 +111,8 @@ func caninl(fn *Node) {
 		return
 	}
 
-	// If marked "go:cgo_unsafe_args", don't inline
+	// If marked "go:cgo_unsafe_args", don't inline, since the
+	// function makes assumptions about its argument frame layout.
 	if fn.Func.Pragma&CgoUnsafeArgs != 0 {
 		reason = "marked go:cgo_unsafe_args"
 		return