Factorize docstring

[glemaitre]

We can factorize some documentation once that Substitution is merged.

Candidates are:

• fit
• _sample

mention that y can be (n_samples,) or (n_samples, n_classes)

[glemaitre]

_sample is a private function which will not show its docstring and fit is mainly defined in base. So no need. So we should remove the docstring of _sample