Companion PR for #6564 (#1350)

* Add log_rotation_opt

* Update Cargo lock

* Use SubstrateCli's log-directory instead of re-defining it

polkadot/cli/src/cli.rs

@@ -84,11 +84,9 @@ pub struct RunCmd {
 #[allow(missing_docs)]
 #[derive(Debug, StructOpt)]
 pub struct Cli {
-	#[allow(missing_docs)]
 	#[structopt(subcommand)]
 	pub subcommand: Option<Subcommand>,
 
-	#[allow(missing_docs)]
 	#[structopt(flatten)]
 	pub run: RunCmd,
 }