DOC extend documentation on sample generators

Also allow make_multilabel_classification to return underlying
distributions.
Merge pull request #3001 from jnothman/doc_sample_gens

[MRG] improve documentation on sample generators
Update gaussian_process.py

just being specific as target value instead of value
FIX bug with set_params on minkowski 'p'.

Fixes #2609.

There was a bug that set_params didn't effectively change the value
of minkowski 'p'. Because 'p' was put in self.metric_kwds in
_init_params and then it never changed there.
DOC deprecation of **kwargs in neighbors per 0.18
TST fix doctest for k-NN
Merge pull request #3588 from tejesh95/patch-1

Update gaussian_process.py