Feature request: expose aesthetic section helper

[teunbrand]

Whenever one is writing a new Geom/Stat, it is always a bit of an inconvenience to manually type out the aesthetics section. With the automatic links and recently showing the default value, writing such a section manually is a pain. One could use #' @eval ggplot2:::rd_aesthetics(...) to generate the section, but you're not supposed to use unexported functions.