@brief Create a list of edge definitions to construct a graph.

`graph_module._edgeDefinitions(relation1, relation2, ..., relationN)`

The list of edge definitions of a graph can be managed by the graph module itself.
This function is the entry point for the management and will return the correct list.

@PARAMS

@PARAM{relationX, object, optional}
An object representing a definition of one relation in the graph

@EXAMPLES

@EXAMPLE_ARANGOSH_OUTPUT{generalGraphEdgeDefinitions}
  var graph_module = require("@arangodb/general-graph");
  directed_relation = graph_module._relation("lives_in", "user", "city");
  undirected_relation = graph_module._relation("knows", "user", "user");
  edgedefinitions = graph_module._edgeDefinitions(directed_relation, undirected_relation);
@END_EXAMPLE_ARANGOSH_OUTPUT