Adjust data field width description to clarify what's possible.

[nixjdm]

Related to lektor/lektor#648

https://www.getlektor.com/docs/models/#fields should be tweaked to say what's really going on I think. In practice, the only useful values are things that round to a multiple of 1/12, from 2/12 to 11/12. Everything else is rounded to those values or 1. This is because we're using bootstrap based on 12 columns, and we don't allow the 1/12 column since it's too small.

So I'd suggest actually listing them. This is the total list, in order: 1/6, 3/12, 1/3, 5/12/, 1/2, 7/12, 3/4, 2/3, 5/6, 11/12 - or equivalent fractions that reduce to these.