doc: GraphVertex and GraphEdge can be erased by calling free()

[ci skip]

doc/Graph.xml

@@ -207,13 +207,15 @@
 			<argument index="0" name="edge" type="GraphEdge" />
 			<description>
 				Removes an edge from the graph. If the graph is simple, you could find an edge with [method find_edge] first, and then remove it.
+				Alternatively, you can also remove the edge by calling [method Object.free] on it.
 			</description>
 		</method>
 		<method name="remove_vertex">
 			<return type="void" />
 			<argument index="0" name="vertex" type="GraphVertex" />
 			<description>
 				Removes the specified vertex from the graph. All edges that are connected to the vertex will be automatically deleted.
+				Alternatively, you can also remove the vertex by calling [method Object.free] on it.
 			</description>
 		</method>
 		<method name="set_iterator_bfs">

doc/GraphEdge.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		An edge connects two [GraphVertex] vertices together. The edge's direction is determined by whether it was added via [method Graph.add_edge] or [method Graph.add_directed_edge]. If the edge is directed, then [member a] and [member b] must be interpreted as endpoints of an arrow pointing [i]from[/i] a vertex [i]to[/i] another.
+		You can call [method Object.free] to remove this edge from the [Graph] it belongs to.
 	</description>
 	<tutorials>
 	</tutorials>

doc/GraphVertex.xml

@@ -20,6 +20,7 @@
 		    print(e.a, " ", e.b)
 		[/codeblock]
 		If you need to traverse the graph without producing duplicates, you may also consider using graph's default [member Graph.iterator].
+		You can call [method Object.free] to remove this vertex from the [Graph] it belongs to.
 	</description>
 	<tutorials>
 	</tutorials>