docs: document negative/infinite order neighborhood (igraph#815)

* docs: document negative/infinite order neighborhood
after the corresponding change in the C core

* chore: update C core

Author: szhorvat

src/_igraph/graphobject.c

@@ -15854,7 +15854,8 @@ struct PyMethodDef igraphmodule_Graph_methods[] = {
    "@param vertices: a single vertex ID or a list of vertex IDs, or\n"
    "  C{None} meaning all the vertices in the graph.\n"
    "@param order: the order of the neighborhood, i.e. the maximum number of\n"
-   "  steps to take from the seed vertex.\n"
+   "  steps to take from the seed vertex. Negative values are interpreted as\n"
+   "  an infinite order, i.e. no limit on the number of steps.\n"
    "@param mode: specifies how to take into account the direction of\n"
    "  the edges if a directed graph is analyzed. C{\"out\"} means that\n"
    "  only the outgoing edges are followed, so all vertices reachable\n"
@@ -15882,7 +15883,8 @@ struct PyMethodDef igraphmodule_Graph_methods[] = {
    "@param vertices: a single vertex ID or a list of vertex IDs, or\n"
    "  C{None} meaning all the vertices in the graph.\n"
    "@param order: the order of the neighborhood, i.e. the maximum number of\n"
-   "  steps to take from the seed vertex.\n"
+   "  steps to take from the seed vertex. Negative values are interpreted as\n"
+   "  an infinite order, i.e. no limit on the number of steps.\n"
    "@param mode: specifies how to take into account the direction of\n"
    "  the edges if a directed graph is analyzed. C{\"out\"} means that\n"
    "  only the outgoing edges are followed, so all vertices reachable\n"