Fix epage's nits

* Use real newlines in command descriptions
* Make `--dest-dir` optional
* Show help message when no subcommand is supplied

Author: notriddle

Diff for: src/tools/rustbook/src/main.rs

@@ -12,15 +12,17 @@ fn main() {
     let crate_version = format!("v{}", crate_version!());
     env_logger::Builder::from_env(env_logger::Env::default().default_filter_or("warn")).init();
     let d_arg = arg!(-d --"dest-dir" <DEST_DIR>
-"The output directory for your book{n}(Defaults to ./book when omitted)");
+"The output directory for your book\n(Defaults to ./book when omitted)")
+    .required(false);
     let dir_arg = arg!([dir]
-"A directory for your book{n}(Defaults to Current Directory when omitted)");
+"A directory for your book\n(Defaults to Current Directory when omitted)");
 
     let matches = Command::new("rustbook")
         .about("Build a book with mdBook")
         .author("Steve Klabnik <[email protected]>")
         .version(&*crate_version)
         .subcommand_required(true)
+        .arg_required_else_help(true)
         .subcommand(
             Command::new("build")
                 .about("Build the book from the markdown files")