snapshots: document use of ioutil.TempDir() (#8508)

Document with inline comments the changes introduced
in https://github.com/cosmos/cosmos-sdk/pull/8475.

Run make format.

server/grpc/grpc_web.go

@@ -3,9 +3,10 @@ package grpc
 import (
 	"net/http"
 
-	"github.com/cosmos/cosmos-sdk/server/config"
 	"github.com/improbable-eng/grpc-web/go/grpcweb"
 	"google.golang.org/grpc"
+
+	"github.com/cosmos/cosmos-sdk/server/config"
 )
 
 // StartGRPCWeb starts a gRPC-Web server on the given address.