explain since format and give examples

Signed-off-by: tronicum <tronicum@user.github.com>

cli/command/container/logs.go

@@ -44,7 +44,7 @@ func NewLogsCommand(dockerCli *command.DockerCli) *cobra.Command {
 
 	flags := cmd.Flags()
 	flags.BoolVarP(&opts.follow, "follow", "f", false, "Follow log output")
-	flags.StringVar(&opts.since, "since", "", "Show logs since timestamp")
+	flags.StringVar(&opts.since, "since", "", "Show logs since timestamp (e.g. 2013-01-02T13:23:37) or relative (e.g. 42m for 42 minutes)")
 	flags.BoolVarP(&opts.timestamps, "timestamps", "t", false, "Show timestamps")
 	flags.BoolVar(&opts.details, "details", false, "Show extra details provided to logs")
 	flags.StringVar(&opts.tail, "tail", "all", "Number of lines to show from the end of the logs")

docs/reference/commandline/logs.md

@@ -24,8 +24,8 @@ Options:
       --details        Show extra details provided to logs
   -f, --follow         Follow log output
       --help           Print usage
-      --since string   Show logs since timestamp
-      --tail string    Number of lines to show from the end of the logs (default "all")
+      --since string   Show logs since timestamp (e.g. 2013-01-02T13:23:37) or relative (e.g. 42m for 42 minutes)
+      --tail string    Number of lines to show from the end of the logs (default "all") 
   -t, --timestamps     Show timestamps
 ```