Add note for Sketchup.active_model.layers[0] #display_name vs #name difference

[MSP-Greg]

With SU 2020 and later:

layer0 = Sketchup.active_model.layers[0]
layer0.name           #=> "Layer0"
layer0.display_name   #=> "Untagged"

Maybe add a note to Sketchup::Layer#display_name and Sketchup::Layer#name to clarify...

Additionally:

1. Sketchup.active_model.layers['Untagged'] == Sketchup.active_model.layers['Untagged'] #=> true
2. It seems that Layer#display_name and Layer#name are identical, with the exception of Layers[0]. Not.sure.