Auto merge of rust-lang#12303 - jonas-schievink:improve-generate-docs-assist, r=jonas-schievink

feat: Improve docs generation assist

- Split into 2 assists: one generates basic docs structure, but no example, one generates an example
- Fix section ordering (the example comes last)
- Allow generating docs for private functions
- Expand `len` to `length` when generating an intro sentence

Author: bors