Updating docblock and manual of Image::toFile method

This 
- updates docblock about using the `Image::toFile()` method to output the image stream directly (ref: [Framework discussion ](https://groups.google.com/forum/#!topic/joomla-dev-framework/qEg7SPyo34I))
- adds `toFile` method to the manual.

Sidenote: I'd like to update the manual to current state of package in other PR, after this is merged.