[MRG] Added note to advice users not to __init__ params with trailing _ in custom defined estimators

Motivated by #3937 

Added a note to [developers/#parameters-and-init](http://scikit-learn.org/dev/developers/#parameters-and-init) section discouraging users not to define parameters with trailing `_` in the `__init__`.

@larsmans Could you take a look?
`if the estimator has been fitted.`