Document vector passing to nudge_y/nudge_x/position_nudge

[MaxAtoms]

Recently, when creating plots with ggstatsplot (using the ggbetweenstats function), I wanted to move the centrality labels for each distribution individually. It took me quite some time to figure out that I can pass a vector to nudge_y (or position_nudge for that matter).

Therefore, I propose to document this functionality.

[teunbrand]

Thank you for the PR! We generally don't want to encourage passing aesthetics outside aes() as vectors for the reasons documented in most layers like ?geom_point:

Please note that while passing unmapped aesthetics as vectors is technically possible, the order and required length is not guaranteed to be parallel to the input data.

If you want to pass it like aes(nudge_y = c(...)) that'd be fine with me. If you intend to change the example, would you mind running devtools::document() so that the changes propagate to the .Rd files?