Fix missing package anchors

[teunbrand]

This PR fixes some CRAN checks:
https://cran.r-project.org/web/checks/check_results_ggalt.html

In particular the following:

Result: NOTE
Found the following Rd file(s) with Rd \link{} targets missing package
anchors:
geom_bkde.Rd: aes, aes_, ggplot, fortify, borders, layer,
geom_histogram, geom_freqpoly, geom_violin
geom_bkde2d.Rd: aes, aes_, ggplot, fortify, borders, layer,
stat_contour, geom_contour, stat_sum
geom_cartogram.Rd: aes, aes_, ggplot, fortify, layer, borders
geom_dumbbell.Rd: aes, aes_, ggplot, fortify, layer, borders
geom_lollipop.Rd: aes, aes_, ggplot, fortify, layer, borders
geom_stateface.Rd: aes, aes_, ggplot, fortify, layer, borders
geom_xspline.Rd: aes, aes_, ggplot, fortify, borders, layer
geom_xspline2.Rd: aes, aes_, ggplot, fortify, borders, layer
stat_ash.Rd: aes, aes_, ggplot, fortify, borders, layer
stat_stepribbon.Rd: aes, aes_, ggplot, fortify, borders, layer
Please provide package anchors for all Rd \link{} targets not in the
package itself and the base packages.

As well as a few other notes.