Mention that neighbors() and shortest_path() may return null-items.

Documentation/Books/Users/Aql/GraphFunctions.mdpp

@@ -8,7 +8,9 @@ instead.
 
 !SECTION Determining direct connections
 
-- *EDGES(edgecollection, startvertex, direction, edgeexamples, options)*:
+!SUBSECTION Edges
+*EDGES(edgecollection, startvertex, direction, edgeexamples, options)*:
+
 Return all edges connected to the vertex *startvertex* as an array. The possible values for
 *direction* are:
   - *outbound*: Return all outbound edges
@@ -35,7 +37,9 @@ instead.
       EDGES(friendrelations, "friends/john", "any", null, {includeVertices: true})
 
 
-- *NEIGHBORS(vertexcollection, edgecollection, startvertex, direction, edgeexamples, options)*:
+!SUBSECTION Neighbors
+*NEIGHBORS(vertexcollection, edgecollection, startvertex, direction, edgeexamples, options)*:
+
 Returns `_id` values of all distinct neighbors that are directly connected to the
 vertex *startvertex* as an array.
 
@@ -70,7 +74,8 @@ instead.
 
 !SECTION General-purpose traversals
 
-- *TRAVERSAL(vertexcollection, edgecollection, startVertex, direction, options)*: 
+!SUBSECTION Traversal
+*TRAVERSAL(vertexcollection, edgecollection, startVertex, direction, options)*: 
 Traverses the graph described by *vertexcollection* and *edgecollection*, 
 starting at the vertex identified by id *startVertex*. Vertex connectivity is
 specified by the *direction* parameter:
@@ -317,8 +322,8 @@ instead.
         data: { attributes: [ "_id", "_key", "name" ] }
       })
 
-
-- *TRAVERSAL_TREE(vertexcollection, edgecollection, startVertex, direction, connectName, options)*: 
+!SUBSECTION Traversal Tree
+*TRAVERSAL_TREE(vertexcollection, edgecollection, startVertex, direction, connectName, options)*: 
 Traverses the graph described by *vertexcollection* and *edgecollection*, 
 starting at the vertex identified by id *startVertex* and creates a hierarchical result.
 Vertex connectivity is establish by inserted an attribute which has the name specified via
@@ -335,8 +340,8 @@ instead.
         itemOrder: "forward"
       })
 
-
-- *SHORTEST_PATH(vertexcollection, edgecollection, startVertex, endVertex, direction, options)*: 
+!SUBSECTION Shortest Path
+*SHORTEST_PATH(vertexcollection, edgecollection, startVertex, endVertex, direction, options)*: 
 Determines the first shortest path from the *startVertex* to the *endVertex*.
 Both vertices must be present in the vertex collection specified in *vertexcollection*,
 and any connecting edges must be present in the collection specified by *edgecollection*.
@@ -474,10 +479,10 @@ instead.
         return (edge.underConstruction === false); // don't follow these edges
       }, false);
 
- 
 !SECTION Other functions
 
-- *PATHS(vertexcollection, edgecollection, direction, options)*: 
+!SUBSECTION Paths
+*PATHS(vertexcollection, edgecollection, direction, options)*: 
 returns an array of paths through the graph defined by the nodes in the collection 
 *vertexcollection* and edges in the collection *edgecollection*. For each vertex
 in *vertexcollection*, it will determine the paths through the graph depending on the
@@ -507,6 +512,18 @@ instead.
         FILTER p.source._id == "users/123456"
         RETURN p.vertices[*].name
 
+!SECTION Graph consistency
+When [using the graph management functions to remove vertices](../GeneralGraphs/Management.md#remove-a-vertex)
+you have the guaranty that all referencing edges are also removed.
+However, if you use document features alone to remove vertices, no edge collections will be adjusted.
+This results in an edge with its  `_from` or `_to` attribute referring to vanished vertices.
+
+Now we query such a graph using the [neighbors](#neighbors) or the [shortest path](#shortest-path) functions.
+If *includeData* wasn't enabled, the referred missing vertex will not be queried, and thus the result
+set will contain the referral to this missing vertex.
+If *includeData* is enabled, these missing vertices will be touched by the query.
+
+In order to keep the result set consistent between *includeData* enabled or disabled, a `null` will be padded to fill the gap for each missing vertex.
 
 !SECTION Performance notes 
 
@@ -525,3 +542,5 @@ process is specified by the optional *maxIterations* configuration value. If the
 vertices processed in a traversal reaches this cap will, the traversal will abort and throw
 a *too many iterations* exception.
 
+
+