Update doc for the --chain flag (#7520)

substrate/client/cli/src/params/shared_params.rs

@@ -23,7 +23,10 @@ use structopt::StructOpt;
 /// Shared parameters used by all `CoreParams`.
 #[derive(Debug, StructOpt)]
 pub struct SharedParams {
-	/// Specify the chain specification (one of dev, local, or staging).
+	/// Specify the chain specification.
+	///
+	/// It can be one of the predefined ones (dev, local, or staging) or it can be a path to a file with
+	/// the chainspec (such as one exported by the `build-spec` subcommand).
 	#[structopt(long, value_name = "CHAIN_SPEC")]
 	pub chain: Option<String>,