Clarify that `fullrange = TRUE` extends range to the axis limits, not the whole plot.

[gregorp]

There's some understandable confusion about geom_smooth(fullrange = TRUE) not making the smooth line go all the way to the axis lines. (Two example posts on Stack Overflow: 1, 2)

The current argument description in ?geom_smooth is:

fullrange Should the fit span the full range of the plot, or just the data?

I suggest modifying the documentation for clarity, something like this:

fullrange If TRUE, the fit will extend to the axis limits, potentially beyond the data. (It will not extend into the padding of the limits configured by expansion().)