DOC clarification of parameter search

The `grid_search` documentation is too centered on grid search, and does not describe what parameter space can be tuned (i.e. mentioning `get_params`). This patches up the documentation by firstly outlining the general idea of parameter search, and by making it more consistent in its use of terminology. It refers to _search candidates_ being one setting of parameters, sampled from a _parameter space_ by the _search method_. It eschews use of _hyperparameters_ because of the use of `get_params`, `set_params` and the _Parameters_ heading in `Estimator` docstrings, but it explains the term.