Update DataFrame.jl

improve docs

Author: emfeltham

src/DataFrame.jl

@@ -6,8 +6,11 @@ import DataFrames.DataFrame
 Construct a DataFrame from a MetaGraph from either its node or edge properties.
 
 `gr` is a MetaGraph.
+
+Optional keyword arguments:
+
 `type` is a Symbol valued either :node or :edge such that the DataFrame is populated with node or edge
-properties stored in `gr`.
+properties stored in `gr`. Default is :node.
 
 """
 function DataFrame(gr::T; type = :node) where T <:AbstractMetaGraph