Reformat Part of 1.7 Release Notes
Reformat Part of 1.7 Release Notes

- Use unordered nested list for stabilised APIs to improve readability
- Add link to `u32::checked_neg`
- Remove trailing back tick from BTreeMap line

GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust