docs: clarify 'Table.read_rows' snippet (#50)

Co-authored-by: kolea2 <45548808+kolea2@users.noreply.github.com>
Co-authored-by: Tres Seaver <tseaver@palladion.com>

docs/snippets_table.py

@@ -202,9 +202,15 @@ def test_bigtable_write_read_drop_truncate():
 
     # Read full table
     partial_rows = table.read_rows()
-    read_rows = [row for row in partial_rows]
+
+    # Read row's value
+    total_rows = []
+    for row in partial_rows:
+        cell = row.cells[COLUMN_FAMILY_ID][col_name][0]
+        print(cell.value.decode("utf-8"))
+        total_rows.append(cell)
     # [END bigtable_read_rows]
-    assert len(read_rows) == len(rows)
+    assert len(total_rows) == len(rows)
     # [START bigtable_drop_by_prefix]
     from google.cloud.bigtable import Client