[Rd] grDevices::grey could provide clearer error message when length(alpha) != length(level)

[Hugh Parsonage]

e.g.

grDevices::grey(level = 0.1, alpha = c(0, 1))
#> Error in grey(level = 0.1, alpha = c(0, 1)) :
#>  attempt to set index 1/1 in SET_STRING_ELT

Perhaps
#> Error in grey(level = 0.1, alpha = c(0, 1)) :
#>  lengths of 'level' and 'alpha' differ