render multi-line document properly

and some clippy fix.

will need a

[ ] Accept embassy-rs/chiptool#31
[ ] Rebase to remove my fork from codebase.

This PR doesn't change function of stm32-data-gen, so there should be no change in ci diff.

BELOW IS OUT OF DATE, SEE embassy-rs/chiptool#31 INSTEAD

Render metapac document from one-line to multi-line.

eg.

from

#[doc = "Scaling factor. Input value has been multiplied by 2^(-n) before for argument. Output value will need to be multiplied by 2^n later for results."]

#[doc = "Scaling factor.  "]
#[doc = "Input value has been multiplied by 2^(-n) before for argument.  "]
#[doc = "Output value will need to be multiplied by 2^n later for results."]

after

embassy-rs / stm32-data

render multi-line document properly #476