add doc in mdbook.md

Author: Shogo Takata

guide/src/format/mdbook.md

@@ -41,7 +41,7 @@ println!("Hello, World!");
 
 If there is no `main` function, then the code is automatically wrapped inside one.
 
-If you wish to disable the play button, you can include the `noplayground` option on the code block like this:
+If you wish to disable the play button for a code block, you can include the `noplayground` option on the code block like this:
 
 ~~~markdown
 ```rust,noplayground
@@ -51,6 +51,13 @@ println!("Hello {}!", name);
 ```
 ~~~
 
+Or, if you wish to disable the play button for all code blocks in your book, you can write the config to the `book.toml` like this.
+
+```toml
+[output.html.playground]
+runnable = false
+```
+
 ## Rust code block attributes
 
 Additional attributes can be included in Rust code blocks with comma, space, or tab-separated terms just after the language term. For example: