docs(update_plot): Remove @keywords internal

[schloerke]

If this method is to be leveraged, it should be public.

{GGally} is looking forward to this method

[schloerke]

/document

[teunbrand]

The update_plot() generic is a workaround for an S7 issue (RConsortium/S7#543).
I'm not sure we'd want to advertise its usage if S7 allows S3 methods for S7 generics as mentioned in the issue.
If S7 solves the issue, we can all stick to ggplot_add() and no migration is necessary.

[schloerke]

Ah! I had missed that we can no override the + with S7 double dispatch. 🎉🎉

ggplot2/R/plot-construction.R

Lines 68 to 76 in a2d0181

	S7::method(`+`, list(class_ggplot, S7::class_any)) <- function(e1, e2) {
	e2name <- deparse(substitute(e2, env = caller_env(2)))
	add_ggplot(e1, e2, e2name)
	}
	S7::method(`+`, list(class_theme, S7::class_any)) <- function(e1, e2) {
	e2name <- deparse(substitute(e2, env = caller_env(2)))
	add_theme(e1, e2, e2name)
	}

Related: Double dispatch with an S3 object in the first slot isn't dispatching properly: #6504

---

Slightly related:
GGally would no longer need +.gg method now that ggplot2 v4 leverages S7. However, GGally only needs ggplot2 >= v3.5.

To get around this, GGally can conditionally export it's +.gg method depending on the ggplot2 version. Similar to https://github.com/RConsortium/S7/blob/db5964265a88d114fb6de97361060979ad63b3d6/vignettes/packages.Rmd#L62-L66 , but with #' @rawNamespace if (packageVersion("ggplot2") < "3.5.2.9001") S3method("+",gg)

[schloerke]

Closing as PR isn't needed