ARROW-2080: [Python] Update documentation about pandas serialization context.

Author: Robert Nishihara <robertnishihara@gmail.com>

Closes #1547 from robertnishihara/serializationdoc and squashes the following commits:

870ff6f [Robert Nishihara] Update documentation about pandas serialization context.

Author: robertnishihara

python/doc/source/ipc.rst

@@ -318,16 +318,16 @@ Serializing pandas Objects
 --------------------------
 
 We provide a serialization context that has optimized handling of pandas
-objects like ``DataFrame`` and ``Series``. This is the
-``pyarrow.pandas_serialization_context`` member. Combined with component-based
+objects like ``DataFrame`` and ``Series``. This can be created with
+``pyarrow.pandas_serialization_context()``. Combined with component-based
 serialization above, this enables zero-copy transport of pandas DataFrame
 objects not containing any Python objects:
 
 .. ipython:: python
 
    import pandas as pd
    df = pd.DataFrame({'a': [1, 2, 3, 4, 5]})
-   context = pa.pandas_serialization_context
+   context = pa.pandas_serialization_context()
    serialized_df = context.serialize(df)
    df_components = serialized_df.to_components()
    original_df = context.deserialize_components(df_components)