Mention missing file creation in build/watch/serve's docs

guide/src/cli/build.md

@@ -7,7 +7,8 @@ mdbook build
 ```
 
 It will try to parse your `SUMMARY.md` file to understand the structure of your
-book and fetch the corresponding files.
+book and fetch the corresponding files. Note that files mentioned in `SUMMARY.md`
+but not present will be created.
 
 The rendered output will maintain the same directory structure as the source for
 convenience. Large books will therefore remain structured when rendered.

guide/src/cli/serve.md

@@ -8,7 +8,8 @@ mdbook serve
 ```
 
 The `serve` command  watches the book's `src` directory for
-changes, rebuilding the book and refreshing clients for each change. A websocket
+changes, rebuilding the book and refreshing clients for each change; this includes
+re-creating deleted files still mentioned in `SUMMARY.md`! A websocket
 connection is used to trigger the client-side refresh.
 
 ***Note:*** *The `serve` command is for testing a book's HTML output, and is not

guide/src/cli/watch.md

@@ -3,7 +3,8 @@
 The `watch` command is useful when you want your book to be rendered on every
 file change. You could repeatedly issue `mdbook build` every time a file is
 changed. But using `mdbook watch` once will watch your files and will trigger a
-build automatically whenever you modify a file.
+build automatically whenever you modify a file; this includes re-creating
+deleted files still mentioned in `SUMMARY.md`!
 
 #### Specify a directory