docs: fix malformed RST formatting in kafka instrumentation docstrings (#4613)

* docs: fix malformed RST in kafka instrumentation docstrings

Fix broken reStructuredText formatting in aiokafka, confluent-kafka, and
kafka-python instrumentation module docstrings (closes #3061):

- Convert plain-text parameter lists to proper RST bullet lists
- Wrap parameter names in double backticks and bold markers
- Add proper RST code blocks for function signature examples
- Fix "API" section underline from ___ to --- (correct RST heading level)

* docs: add changelog fragment for #4613

* chore: add missing newline at end of changelog file

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>