Make the description of `multiple` keywork clearer

The "default" description does not appear in the same color as set, form and display. So at first I thought that I had to use set-multiple-...

Put it in bold can make it cleared that this is not a part of the set section ?

Or perhaps move that part about default at the top ?

Second change : I did not understand the sentence "The fields to display are chosen with the fields attribute." So I changed it to make it more clear what the "fields attribute" is.