Mention grDevices as the location where devices come from

[DavisVaughan]

My frustrated experience using gganimate as someone who doesn't do this kind of thing very often:

• I exported an image to .gif using anim_save()

• The image looked pretty grainy (text and lines), and was 480x480

• I wanted to make it larger, and higher resolution, so I looked at the docs for ?animate() and for ?anim_save()

• I see that the gifski_renderer() is the default, and has a width / height param, so I try setting that but it doesn't seem to change much.

• I see that there is a device argument for animate(), and that ... is passed on to that. It says it is defaulted to "png" but I have no idea what function it is calling under the hood, so I don't know what I can pass through here.

• I remember that the ?gifski_renderer docs actually say it requires the png package, so I look at png:: to see what functions are there but those don't seem useful either.

• Finally I ask Thomas, and learn that I can pass width and height directly to animate() because they get passed through to grDevices::png().

Summary:

I got really confused, but would have greatly benefited from a link to grDevices::png in the devices argument in animate()