Improved docstring for predictions argument in sample_posterior_predictive (#6616)

* Improved docstring for predictions argument in sample_posterior_predictive

* Fix typos

Co-authored-by: Ravin Kumar <7213793+canyon289@users.noreply.github.com>

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Co-authored-by: Ravin Kumar <7213793+canyon289@users.noreply.github.com>

Author: fonnesbeck

pymc/sampling/forward.py

@@ -481,8 +481,7 @@ def sample_posterior_predictive(
         Whether to automatically use :meth:`arviz.InferenceData.extend` to add the posterior predictive samples to
         ``trace`` or not. If True, ``trace`` is modified inplace but still returned.
     predictions : bool, default False
-        Choose the function used to convert the samples to inferencedata. See ``idata_kwargs``
-        for more details.
+        Flag used to set the location of posterior predictive samples within the returned ``arviz.InferenceData`` object. If False, assumes samples are generated based on the fitting data to be used for posterior predictive checks, and samples are stored in the ``posterior_predictive``. If True, assumes samples are generated based on out-of-sample data as predictions, and samples are stored in the ``predictions`` group.
     idata_kwargs : dict, optional
         Keyword arguments for :func:`pymc.to_inference_data` if ``predictions=False`` or to
         :func:`pymc.predictions_to_inference_data` otherwise.