I think this docstring is not explicit enough: what about something like:

```
"""Incrementally fit new trees and add them to the previously fitted ensemble.

partial_fit will not grow any new tree if the currently existing ensemble of trees
stored in the ``estimators_`` attribute already has size `n_estimators`. Setting a larger value
for ``n_estimators`` with a statement like ``model.set_params(n_estimators=2 * model.n_estimators)``
is likely to be necessary for this call to have any effect on the predictive accuracy of the
model after the first call.

Contrary to other implementations of the `partial_fit` method in other models this method is not
meant to chunk a large scale training dataset to implement out-of-core learning.

The output of the decision function of the past trees applied on the newly provided X is used
to compute the boosting weights when fitting the new trees.

"""
```