add text to docstring in divergence.py
add text to docstring in divergence.py

cmshobe/landlab,csherwood-usgs/landlab,ManuSchmi88/landlab,cmshobe/landlab,laijingtao/landlab,Carralex/landlab,amandersillinois/landlab,RondaStrauch/landlab,amandersillinois/landlab,landlab/landlab,RondaStrauch/landlab,landlab/landlab,ManuSchmi88/landlab,landlab/landlab,cmshobe/landlab,SiccarPoint/landlab,Carralex/landlab,Carralex/landlab,csherwood-usgs/landlab,laijingtao/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,SiccarPoint/landlab