Documentation fixes for `coord_radial()`.

1. The placement of "superseded" here seems misleading, as it sits next to geom_radial(), which actually supersedes geom_polar().

2. Is the behaviour of the last example (code below) doing what you want it to do? (There are points displayed outside the "plot area", for example.) Or is the intention to have something more like the image below (taken from a Plotnine example here).

3. The examples don't fully exercise geom_radial(). I collected some examples (for Plotnine) here. I understand that might be too elaborate for the documentation.

ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  coord_radial(
    start = -0.4 * pi,
    end = 0.4 * pi, inner.radius = 0.3,
    thetalim = c(200, 300),
    rlim = c(15, 30),
  )

Start with the coordinate-systems reference page linked in the issue and compare its coord_radial() and geom_radial() entries with the supplied ggplot example and linked Plotnine examples. Clarify the intended plot-area behavior, correct the placement of “superseded,” and revise the examples so they better exercise geom_radial().