Document flipped computed variables

[yutannihilation]

Fix #4124

These four stat have variables whose names differ depending on the orientation.

• stat_boxplot(): middle, lower, and upper. This feels a bit tricky at least in two points; the flipped names are asynmetric (e.g. middle vs xmiddle, not ymiddle vs xmiddle), and notchlower and notchupper doesn't have their flipped form.
• stat_smooth(): y, ymin, and ymax
• stat_function() stat_ecdf() : these generate both x and y, so the documentation would be cryptic like below, so I don't address them in this pull request (at least they don't have undocumented computed variables)

#' `stat_function()` computes the following variables:
#' \describe{
#'   \item{x or y}{x or y values along a grid}
#'   \item{y or x}{value of the function evaluated at corresponding x or y}
#' }

edit: I just remembered stat_ecdf() doesn't support flipping (c.f. #4005)

[yutannihilation]

Added this sentence. Does this sound good?

stat_boxplot() provides the following variables, some of which depend on the orientation:

[clauswilke]

I think this is fine. Maybe put the "or" in italics just as is the case for the aesthetics.

[clauswilke]

The "or" between computed variables, I mean.

[yutannihilation]

Makes sense, I put them in Italic.

[clauswilke]

LGTM

[clauswilke]

Oh, this was merged already. Looks like my browser had cached an old version.