DOCS: Improve description of estimate parameter

[sebp]

Checklist

• [x] closes #414 and #416
• [x] pytest passes
• [x] code is well formatted
• [x] documentation renders correctly

What does this implement/fix? Explain your changes Improves the description of the estimate parameter in brier_score and integrated_brier_score.

[codecov[bot]]

Codecov Report

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

Comparison is base (ef6e9c3) 97.98% compared to head (8d7b855) 97.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #424 +/- ## ======================================= Coverage 97.98% 97.98% ======================================= Files 37 37 Lines 3426 3426 Branches 517 517 ======================================= Hits 3357 3357 Misses 33 33 Partials 36 36 ```

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