Add some explanations about bbox = "FULL"

I've used this option a few times but I don't think it's documented anywhere. For example:

```
library(tmap)

tm_shape(World, 
         bbox = "FULL",
         crs = "+proj=ortho +lat_0=30 +lon_0=0") +
  tm_polygons() +
  tm_graticules()
```

Is this identical to `bbox = sf::st_bbox()`?