[ENH] Improve `_check_params` method in `_k_means.py` and `_k_medoids.py`

[tanishy7777]

Describe the feature or idea you want to propose

Right now there is no else condition in the _check_params function if the self.init is a string.

def _check_params(self, X: np.ndarray) -> None:
        self._random_state = check_random_state(self.random_state)

        if isinstance(self.init, str):
            if self.init == "random":
                self._init = self._random_center_initializer
            elif self.init == "kmedoids++":
                self._init = self._kmedoids_plus_plus_center_initializer
            elif self.init == "first":
                self._init = self._first_center_initializer
            elif self.init == "build":
                self._init = self._pam_build_center_initializer
        else:
          # other code

So when we pass a string other than the expected strings it leads to an error which is not very user friendly.
Would be nice to let the user know what the cause of the error is.

Also noticed that the build initialization doesn't have any documentation. Would be good to at that as well?

Describe your proposed solution

add an else statement

Describe alternatives you've considered, if relevant

No response

Additional context

No response