docs(api.md): Added cutoff values for scale option in page.pdf functi…

…on (#3201)

Added the cutoff values for the scale option in the page.pdf function call. Numbers based on chrome's warning message.

docs/api.md

@@ -1411,7 +1411,7 @@ Page is guaranteed to have a main frame which persists during navigations.
 #### page.pdf(options)
 - `options` <[Object]> Options object which might have the following properties:
   - `path` <[string]> The file path to save the PDF to. If `path` is a relative path, then it is resolved relative to [current working directory](https://nodejs.org/api/process.html#process_process_cwd). If no path is provided, the PDF won't be saved to the disk.
-  - `scale` <[number]> Scale of the webpage rendering. Defaults to `1`.
+  - `scale` <[number]> Scale of the webpage rendering. Defaults to `1`. Scale amount must be between 0.1 and 2.
   - `displayHeaderFooter` <[boolean]> Display header and footer. Defaults to `false`.
   - `headerTemplate` <[string]> HTML template for the print header. Should be valid HTML markup with following classes used to inject printing values into them:
     - `date` formatted print date