Improve documentation of arguments to xlim/ylim

[jarauh]

Currently, xlim and ylim are documented together with lims. Thus, the argument ... to all three functions is described as:

A name-value pair. The name must be an aesthetic, and the value must be either a length-2 numeric, a character, a factor, or a date/time.

Which is not correct for xlim/ylim.

Suggested text:

For `xlim` and `ylim`: Two numeric values, specifying the left/lower limit and the right/upper limit of the scale. If the larger value comes first, the scale will be reversed. You can leave one value as NA to compute from the range of the data.

For `lims`: A name-value pair. The name must be an aesthetic, and the value must be either a length-2 numeric, a character, a factor, or a date/time.

A numeric value will create a continuous scale. If the larger value comes first, the scale will be reversed. You can leave one value as NA to compute from the range of the data.

A character or factor value will create a discrete scale.

A date-time value will create a continuous date/time scale.

Alternative would be to move some of this to the details section.