too_long_first_document should have a MaybeIncorrect suggestion for splitting out the first sentence of docs

[Manishearth]

too_long_first_document is a new lint that would ideally be more automatable. Currently it suggests adding empty lines when the first doc line ends in a period, but I wonder if we should also have it suggest splitting the docs after the first sentence break (typically a period and a space) as a MaybeIncorrect suggestion that people can __CARGO_FIX_YOLO=1 if they need.