Explicit about single slider in SliderGrid

This PR implements the solution I proposed in [this comment](MakieOrg#2923 (comment)), which was a result of the discussion above.

src/makielayout/types.jl

@@ -944,8 +944,8 @@ end
 end
 
 """
-A grid of horizontal `Slider`s, where each slider has one name label on the left,
-and a value label on the right.
+A grid of one or more horizontal `Slider`s, where each slider has a 
+name label on the left and a value label on the right.
 
 Each `NamedTuple` you pass specifies one `Slider`. You always have to pass `range`
 and `label`, and optionally a `format` for the value label. Beyond that, you can set