gh-133879: Clean up What's New for 3.15 (#140435)

Clean up What's New for 3.15

A bit early but I was reading through it and noticed some issues:

- A few improvements were listed in the removals section
- The "Porting to 3.15" section in the C API chapter had some
  changes that aren't about the C API
- Some other typos and wording fixes