Improved documentation of the "estimator_params" argument for RFE and RFECV.

Changed the documentation for the "estimator_params" argument for both
RFE and RFECV to better indicate how it could be useful for doing a
grid search. Previously the documentation may have indicated that RFE
and RFECV would perform a grid search if "estimator_params" was passed
a grid of parameter values, which is not the actual behavior.
Merge pull request #3208 from staubda/improve_RFE_doc

Improved documentation of the "estimator_params" argument for RFE and RF...