Update documentation for agg_png with ggsave

[jjcrumpler3]

First time I've reported an issue, so please bare with me if I leave anything out. The protocol for saving a plot with ggsave with device = agg_png seems to have changed, but not clearly communicated within documentation.

previous example may be something like

ggsave("mtcars.png", device = agg_png, res = 400)

However, the working method now is

ggsave("mtcars.png", device = agg_png(res = 400))

I like the change, but it does not seem to be documented anywhere, at least nowhere that I noticed.

[thomasp85]

This is not correct. The right way to use ragg with ggsave() is the first mentioned approach... There is, however, some unfortunate interaction between the res argument in ragg_png() and the dpi argument in ggsave()