add irradiance.dni() function to Decomposing and combining irradiance section of api.rst. 
add irradiance.dni() function to Decomposing and combining irradiance section of api.rst. 


pvlib/pvlib-python,cwhanse/pvlib-python,mikofski/pvlib-python,rubennj/pvlib-python,alorenzo175/pvlib-python,anomam/pvlib-python,wholmgren/pvlib-python