Fix _postprocess call in extending doc
Fix _postprocess call in extending doc

mwstobo/marshmallow,0xDCA/marshmallow,Tim-Erwin/marshmallow,dwieeb/marshmallow,marshmallow-code/marshmallow,xLegoz/marshmallow,quxiaolong1504/marshmallow,daniloakamine/marshmallow,VladimirPal/marshmallow,maximkulkin/marshmallow,bartaelterman/marshmallow,etataurov/marshmallow,0xDCA/marshmallow,Bachmann1234/marshmallow