Clarity that path_length is a count, and that distance_between gives …

…distance

python/tskit/trees.py

@@ -2970,13 +2970,16 @@ def rf_distance(self, other):
 
     def path_length(self, u, v):
         """
-        Returns the path length between two nodes
-        (i.e., the number of edges between two nodes in this tree).
+        Returns a count of the number of edges between two nodes in this tree.
         If the two nodes have a most recent common ancestor, then this is defined as
         ``tree.depth(u) + tree.depth(v) - 2 * tree.depth(tree.mrca(u, v))``. If the nodes
         do not have an MRCA (i.e., they are in disconnected subtrees) the path length
         is infinity.
 
+        .. note:: This counts the number of "hops" between two nodes. To find the branch
+            length distance between them, in units of time (i.e. the sum of edge lengths
+            that separate two nodes) use the :meth:`.distance_between` method instead.
+
         .. seealso:: See also the :meth:`.depth` method
 
         :param int u: The first node for path length computation.