Fix BFS Docstring (rapidsai#2318)

Corrects the BFS docstring by clarifying that multiple start vertices are accepted as long as they are from different components, and that passing start vertices from the same component will result in undefined behavior.

Authors:
  - Alex Barghi (https://github.com/alexbarghi-nv)
  - Chuck Hastings (https://github.com/ChuckHastings)

Approvers:
  - https://github.com/betochimas
  - Joseph Nke (https://github.com/jnke2016)
  - Brad Rees (https://github.com/BradReesWork)

URL: rapidsai#2318

python/cugraph/cugraph/dask/traversal/bfs.py

@@ -98,12 +98,14 @@ def bfs(input_graph,
     ----------
     input_graph : cugraph.Graph
         cuGraph graph instance, should contain the connectivity information
-        as dask cudf edge list dataframe(edge weights are not used for this
+        as dask cudf edge list dataframe (edge weights are not used for this
         algorithm).
 
-    start : Integer
-        Specify starting vertex for breadth-first search; this function
-        iterates over edges in the component reachable from this node.
+    start : Integer or list
+        The id of the graph vertex from which the traversal begins, or
+        if a list, the vertex from which the traversal begins in each
+        component of the graph.  Only one vertex per connected
+        component of the graph is allowed.
 
     depth_limit : Integer or None, optional (default=None)
         Limit the depth of the search

python/cugraph/cugraph/traversal/bfs.py

@@ -183,8 +183,11 @@ def bfs(G,
         information. Edge weights, if present, should be single or double
         precision floating point values.
 
-    start : Integer, optional (default=None)
-        The index of the graph vertex from which the traversal begins
+    start : Integer or list, optional (default=None)
+        The id of the graph vertex from which the traversal begins, or
+        if a list, the vertex from which the traversal begins in each
+        component of the graph.  Only one vertex per connected
+        component of the graph is allowed.
 
     depth_limit : Integer or None, optional (default=None)
         Limit the depth of the search