Fix comment about swap limit of docker update

The description "set `-1` to disable swap" is wrong, `build`,
`create` and `run` already fixed, we need to fix `update` as well.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

api/client/update.go

@@ -23,7 +23,7 @@ func (cli *DockerCli) CmdUpdate(args ...string) error {
 	flCPUShares := cmd.Int64([]string{"#c", "-cpu-shares"}, 0, "CPU shares (relative weight)")
 	flMemoryString := cmd.String([]string{"m", "-memory"}, "", "Memory limit")
 	flMemoryReservation := cmd.String([]string{"-memory-reservation"}, "", "Memory soft limit")
-	flMemorySwap := cmd.String([]string{"-memory-swap"}, "", "Total memory (memory + swap), '-1' to disable swap")
+	flMemorySwap := cmd.String([]string{"-memory-swap"}, "", "Swap limit equal to memory plus swap: '-1' to enable unlimited swap")
 	flKernelMemory := cmd.String([]string{"-kernel-memory"}, "", "Kernel memory limit")
 
 	cmd.Require(flag.Min, 1)

docs/reference/commandline/update.md

@@ -23,7 +23,7 @@ parent = "smn_cli"
       --cpuset-mems=""           Memory nodes (MEMs) in which to allow execution (0-3, 0,1)
       -m, --memory=""            Memory limit
       --memory-reservation=""    Memory soft limit
-      --memory-swap=""           Total memory (memory + swap), '-1' to disable swap
+      --memory-swap=""           A positive integer equal to memory plus swap. Specify -1 to enable unlimited swap
       --kernel-memory=""         Kernel memory limit: container must be stopped
 
 The `docker update` command dynamically updates container resources.  Use this