Clarify MultiTopo docstrs and copy addLInk opts

Note: it's a bit confusing, but we need to copy the link
parameter dicts (since we update them with node info), but we
can share the node dicts. Perhaps we should copy the node
dicts as well...

mininet/topo.py

@@ -23,7 +23,8 @@ class MultiGraph( object ):
     def add_node( self, node, attr_dict=None, **attrs):
         """Add node to graph
            attr_dict: attribute dict (optional)
-           attrs: more attributes (optional)"""
+           attrs: more attributes (optional)
+           warning: updates attr_dict with attrs"""
         attr_dict = {} if attr_dict is None else attr_dict
         attr_dict.update( attrs )
         self.node[ node ] = attr_dict
@@ -31,7 +32,9 @@ class MultiGraph( object ):
     def add_edge( self, src, dst, key=None, attr_dict=None, **attrs ):
         """Add edge to graph
            key: optional key
-           attr_dict: optional attribute dict"""
+           attr_dict: optional attribute dict
+           attrs: more attributes
+           warning: udpates attr_dict with attrs"""
         attr_dict = {} if attr_dict is None else attr_dict
         attr_dict.update( attrs )
         self.node.setdefault( src, {} )
@@ -151,6 +154,7 @@ class Topo( object ):
         if not opts and self.lopts:
             opts = self.lopts
         port1, port2 = self.addPort( node1, node2, port1, port2 )
+        opts = dict( opts )
         opts.update( node1=node1, node2=node2, port1=port1, port2=port2 )
         self.g.add_edge(node1, node2, key, opts )
         return key