Improve `fit_all_nuisance` and `fit_all_treatment` docstring

[FrancescMartiEscofetQC]

Checklist

• [ ] Added a CHANGELOG.rst entry

[FrancescMartiEscofetQC]

FYI @ArseniyZvyagintsevQC

[codecov[bot]]

Codecov Report

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

Project coverage is 94.88%. Comparing base (5c8e467) to head (7da126b). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #69 +/- ## ======================================= Coverage 94.88% 94.88% ======================================= Files 15 15 Lines 1544 1544 ======================================= Hits 1465 1465 Misses 79 79 ```

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