[3.12] Docs: Argument Clinic: Improve 'How to write a custom converter' (GH-107328) (#107669)

Docs: Argument Clinic: Improve 'How to write a custom converter' (GH-107328)

- Omit unneccesary wording and sentences
- Don't mention implementation details (no digression, explanation)

(cherry picked from commit 4a5b4221e381c541f3f73537b7b87580d100158b)

Co-authored-by: Erlend E. Aasland <erlend@python.org>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>