doc: update wording in RowIterator docstrings to reduce confusion (#127)

Goal: Talking about partition filtration can be problematic, so refer
to Storage API capabilities more obliquely.  Particularly, partition
filtration is possible via a query or a direct storage API read, so
don't use as severe of language when describing helper mechanisms such
as to_dataframe() on row iterators.

Fixes: googleapis/python-bigquery-storage#22

Co-authored-by: Peter Lamut <plamut@users.noreply.github.com>

google/cloud/bigquery/table.py

@@ -1515,8 +1515,9 @@ def to_arrow(
                 This method requires the ``pyarrow`` and
                 ``google-cloud-bigquery-storage`` libraries.
 
-                Reading from a specific partition or snapshot is not
-                currently supported by this method.
+                This method only  exposes a subset of the capabilities of the
+                BigQuery Storage API.  For full access to all features
+                (projections, filters, snapshots) use the Storage API directly.
             create_bqstorage_client (bool):
                 Optional. If ``True`` (default), create a BigQuery Storage API
                 client using the default API settings. The BigQuery Storage API
@@ -1598,8 +1599,9 @@ def to_dataframe_iterable(self, bqstorage_client=None, dtypes=None):
                 This method requires the ``pyarrow`` and
                 ``google-cloud-bigquery-storage`` libraries.
 
-                Reading from a specific partition or snapshot is not
-                currently supported by this method.
+                This method only  exposes a subset of the capabilities of the
+                BigQuery Storage API.  For full access to all features
+                (projections, filters, snapshots) use the Storage API directly.
 
                 **Caution**: There is a known issue reading small anonymous
                 query result tables with the BQ Storage API. When a problem
@@ -1666,8 +1668,9 @@ def to_dataframe(
                 This method requires the ``pyarrow`` and
                 ``google-cloud-bigquery-storage`` libraries.
 
-                Reading from a specific partition or snapshot is not
-                currently supported by this method.
+                This method only  exposes a subset of the capabilities of the
+                BigQuery Storage API.  For full access to all features
+                (projections, filters, snapshots) use the Storage API directly.
 
                 **Caution**: There is a known issue reading small anonymous
                 query result tables with the BQ Storage API. When a problem