Add explanations, examples.
Add property 'Cancel', 'SuppressDefaultDialog'

- More accurate the description for different Kind, including top-level document, subresources, wiki link.
- Rewrite the sample code. The methods of add event handler were removed. In new sample code, one method will both register handler and call save as.
- Add comment to address get_ContentMimeType can help the Kind selection.
- Add comment to address the CoreWebView2SaveAsUIResults will help end developer know the error if API call failed.