And improve docs, the error message, etc.
I think false is the better default since such errors should only occur due to a bug in Rustfmt and therefore most users should not be notified of it happening (although to be clear, it might be a 'bug' which only occurs with pathological input and therefore we won't fix it). The error has proven to be confusing and annoying in the past.
Closes#1080
f14671131e