Improve quantile docs

[teunbrand]

This PR aims to fix #6641.

In addition to improving the docs around quantiles, it also interprests logical linetype values as numeric.
From this follows that linetype = TRUE becomes solid and linetype = FALSE becomes blank.
It is convenient if you want a logical switch for linetypes, like so:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(mpg, aes(drv, displ)) +
  geom_violin(quantile.linetype = TRUE)

^{Created on 2025-09-26 with reprex v2.1.1}