[MRG+3] Fix min_weight_fraction_leaf to work when sample_weights are not provided (#7301)

* fix min_weight_fraction_leaf when sample_weights is None

* fix flake8 error

* remove added newline and unnecessary assignment

* remove max bc it's implemented in cython and add interaction test

* edit weight calculation formula and add test to check equality

* remove test that sees if two parameter build the same tree

* reword min_weight_fraction_leaf docstring

* clarify uniform weight in forest docstrings

* update docstrings for all classes

* add what's new entry

* move whatsnew entry to bug fixes and explain previous behavior