update doc

docs/using/methods.rst

@@ -13,15 +13,60 @@ returned by ``open_dataset``:
  Methods
 *********
 
-__len__
+__len__()
    The number of rows (dates) in the dataset.
 
-__getitem__
+__getitem__(key)
    Access the dataset's data values.
 
+provenance()
+   Return the dataset's provenance information.
+
+source(index):
+   Return the source of the dataset's data.
+
+tree():
+   Return the dataset's tree.
+
 ************
  Attributes
 ************
 
 shape:
    A tuple of the dataset's dimensions.
+
+dtype:
+   The dataset's data type.
+
+field_shape:
+   The dataset's field shape.
+
+dates:
+   The dataset's dates.
+
+latitudes:
+   The dataset's latitudes.
+
+longitudes:
+   The dataset's longitudes.
+
+statistics:
+   The dataset's statistics.
+
+resolution:
+   The dataset's resolution.
+
+frequency:
+   The dataset's frequency.
+
+name_to_index:
+   A dictionary mapping variable names to their indices.
+
+variables:
+   A list of the dataset's variable names.
+
+missing:
+   The index of the missing dates.
+
+grids:
+   The dataset's grids.