Editing for doc on additive.py and multiplicative.py + tbpcf model

[CamilleDiez]

Editing for cleaner docs for additive and multiplicative models. More detailed description for Tbabs. Added Tbpcf model.

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📚 Documentation preview 📚: https://jaxspec--135.org.readthedocs.build/en/135/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.28%. Comparing base (899de62) to head (6fe4076).

:exclamation: Current head 6fe4076 differs from pull request most recent head 666506d. Consider uploading reports for the commit 666506d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #135 +/- ## ========================================== + Coverage 89.16% 89.28% +0.11% ========================================== Files 17 17 Lines 1034 1045 +11 ========================================== + Hits 922 933 +11 Misses 112 112 ```

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