[MRG] Clarified indempotence of fit (#12305)
[MRG] Clarified indempotence of fit (#12305)

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#### What does this implement/fix? Explain your changes.

As far as I understand it, the fact that `fit` is idempotent means that repeated calls to `fit` with the same data doesn't change the estimator.

The contributing guide was a bit unclear about this.

#### Any other comments?


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