Improvements to docs for MarkovChain

[jstac]

At the moment, with properties, we see no documentation:

In [9]: import quantecon as qe

In [10]: m = qe.MarkovChain([[1, 0], [0, 1]])

In [11]: m.stationary_distributions?
Type:        property
String form: <property object at 0x7f34106c45e8>
Docstring:   <no docstring>

However, as @cc7768 pointed out, it is possible to get documentation to show up here but adding docstrings to the functions associated with the property.