update docs

book-example/src/format/summary.md

@@ -22,15 +22,25 @@ allow for easy parsing. Let's see how you should format your `SUMMARY.md` file.
    [Title of prefix element](relative/path/to/markdown.md)
    ```
 
-3. ***Numbered Chapter*** Numbered chapters are the main content of the book,
+3. ***Part Title:*** An optional title for the next collect of numbered
+   chapters. The numbered chapters can be broken into as many parts as
+   desired.
+
+4. ***Numbered Chapter*** Numbered chapters are the main content of the book,
    they will be numbered and can be nested, resulting in a nice hierarchy
    (chapters, sub-chapters, etc.)
    ```markdown
+   # Title of Part
+
    - [Title of the Chapter](relative/path/to/markdown.md)
+
+   # Title of Another Part
+
+   - [More Chapters](relative/path/to/markdown2.md)
    ```
    You can either use `-` or `*` to indicate a numbered chapter.
 
-4. ***Suffix Chapter*** After the numbered chapters you can add a couple of
+5. ***Suffix Chapter*** After the numbered chapters you can add a couple of
    non-numbered chapters. They are the same as prefix chapters but come after
    the numbered chapters instead of before.
 
@@ -50,5 +60,5 @@ error.
   of contents, as you can see for the next chapter in the table of contents on the left.
   Draft chapters are written like normal chapters but without writing the path to the file
   ```markdown
-  - [Draft chapter]()  
+  - [Draft chapter]()
-  ``` 
+  ```