DOC: suppress some doc build warnings (pandas-dev#15958)

Author: jreback

doc/source/dsintro.rst

@@ -979,6 +979,7 @@ Convert to a MultiIndex DataFrame
 Alternatively, one can convert to an xarray ``DataArray``.
 
 .. ipython:: python
+   :okwarning:
 
    p.to_xarray()
 

doc/source/io.rst

@@ -4487,6 +4487,7 @@ See the `Full Documentation <https://github.com/wesm/feather>`__
 Write to a feather file.
 
 .. ipython:: python
+   :okwarning:
 
    df.to_feather('example.feather')
 

doc/source/whatsnew/v0.13.0.txt

@@ -357,11 +357,11 @@ HDFStore API Changes
   .. ipython:: python
 
      path = 'test.h5'
-     df = DataFrame(randn(10,2))
+     df = pd.DataFrame(np.random.randn(10,2))
      df.to_hdf(path,'df_table',format='table')
      df.to_hdf(path,'df_table2',append=True)
      df.to_hdf(path,'df_fixed')
-     with get_store(path) as store:
+     with pd.HDFStore(path) as store:
         print(store)
 
   .. ipython:: python