vfs,mount,cmount,serve: Add documentation for vfs caching modes

cmd/serve/http/http.go

@@ -45,7 +45,7 @@ The server will log errors.  Use -v to see access logs.
 
 --bwlimit will be respected for file transfers.  Use --stats to
 control the stats printing.
-`,
+` + vfs.Help,
 	Run: func(command *cobra.Command, args []string) {
 		cmd.CheckArgs(1, 1, command, args)
 		f := cmd.NewFsSrc(args)