[ Plasma C++ ] add more docs in python plasma

[asfimport]

add docs about how to get metadata in plasma client and how to use subsribe & notification
function

Reporter: Lu Qi / @luchy0120

_{Note: This issue was originally created as ARROW-1804. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.