update(bin): add doccomments to subcommands

src/main.rs

@@ -15,13 +15,16 @@ struct Args {
 
 #[derive(Debug, Subcommand)]
 enum Commands {
+    /// Generate register interface code.
     Generate {
         /// Output directory.
         out: path::PathBuf,
     },
+    /// Generate flattened register map in CSV.
     #[cfg(feature = "flatmap")]
     Flatmap {
         /// Flattened csv out path.
+        /// Print to stdout by default.
         flatmap: Option<path::PathBuf>,
     },
 }