Adding documentation functionality on the nodes.

public virtual string documentationURL()   has been added to AbstractMaterialNode
Pressing F1 will take you to the documentation if it exists.
Right clicking on a node gives you the context menu item See Documentation that also takes you to the documentation if it exists.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/GraphEditorView.cs

         string m_FloatingWindowsLayoutKey;
         FloatingWindowsLayout m_FloatingWindowsLayout;
 
-        string m_DocumentURL = "https://github.com/Unity-Technologies/ShaderGraph/wiki";
         public Action saveRequested { get; set; }
 
         public Action convertToSubgraphRequested
                     var nodeView = (MaterialNodeView)graphView.selection.First();
                     if(nodeView.node.documentationURL != null)
                         System.Diagnostics.Process.Start(nodeView.node.documentationURL);
-                    else
-                        Debug.Log("Missing Documentation URL for -> " + System.Diagnostics.Process.Start(m_DocumentURL));
                 }
             }
         }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/MaterialGraphView.cs

         public AbstractMaterialGraph graph { get; private set; }
         public Action onConvertToSubgraphClick { get; set; }
 
-        private string m_DocumentURL = "https://github.com/Unity-Technologies/ShaderGraph/wiki";
-
         public override List<Port> GetCompatiblePorts(Port startAnchor, NodeAdapter nodeAdapter)
         {
             var compatibleAnchors = new List<Port>();
                 evt.menu.AppendAction("Convert To Inline Node", ConvertToInlineNode, ConvertToInlineNodeStatus);
                 evt.menu.AppendAction("Convert To Property", ConvertToProperty, ConvertToPropertyStatus);
                 evt.menu.AppendSeparator();
-                evt.menu.AppendAction("See Documentation", SeeDocumentation, SeeDocumentationStatus);
+                evt.menu.AppendAction("Open Documentation", SeeDocumentation, SeeDocumentationStatus);
             }
             else if (evt.target is BlackboardField)
             {
             var node = selection.OfType<MaterialNodeView>().First().node;
             if (node.documentationURL != null)
                 System.Diagnostics.Process.Start(node.documentationURL);
-            else
-                Debug.Log("Missing Documentation URL for -> " + System.Diagnostics.Process.Start(m_DocumentURL));
         }
 
         ContextualMenu.MenuAction.StatusFlags SeeDocumentationStatus(EventBase eventBase)
+            if (selection.OfType<MaterialNodeView>().First().node.documentationURL == null)
+                return ContextualMenu.MenuAction.StatusFlags.Disabled;
             return ContextualMenu.MenuAction.StatusFlags.Normal;
         }