2.13 collections doc: link to Vector changes

_overviews/collections-2.13/concrete-immutable-collection-classes.md

@@ -100,13 +100,14 @@ Vectors are built and modified just like any other sequence.
     scala> vec3(0)
     res1: Int = 100
 
-Vectors are represented as trees with a high branching factor (The branching factor of a tree or a graph is the number of children at each node). Every tree node contains up to 32 elements of the vector or contains up to 32 other tree nodes. Vectors with up to 32 elements can be represented in a single node. Vectors with up to `32 * 32 = 1024` elements can be represented with a single indirection. Two hops from the root of the tree to the final element node are sufficient for vectors with up to 2<sup>15</sup> elements, three hops for vectors with 2<sup>20</sup>, four hops for vectors with 2<sup>25</sup> elements and five hops for vectors with up to 2<sup>30</sup> elements. So for all vectors of reasonable size, an element selection involves up to 5 primitive array selections. This is what we meant when we wrote that element access is "effectively constant time".
+Vectors are represented as trees with a high branching factor. (The branching factor of a tree or a graph is the number of children at each node.) The details of how this is accomplished [changed](https://github.com/scala/scala/pull/8534) in Scala 2.13.2, but the basic idea remains the same, as follows.
+
+Every tree node contains up to 32 elements of the vector or contains up to 32 other tree nodes. Vectors with up to 32 elements can be represented in a single node. Vectors with up to `32 * 32 = 1024` elements can be represented with a single indirection. Two hops from the root of the tree to the final element node are sufficient for vectors with up to 2<sup>15</sup> elements, three hops for vectors with 2<sup>20</sup>, four hops for vectors with 2<sup>25</sup> elements and five hops for vectors with up to 2<sup>30</sup> elements. So for all vectors of reasonable size, an element selection involves up to 5 primitive array selections. This is what we meant when we wrote that element access is "effectively constant time".
 
 Like selection, functional vector updates are also "effectively constant time". Updating an element in the middle of a vector can be done by copying the node that contains the element, and every node that points to it, starting from the root of the tree. This means that a functional update creates between one and five nodes that each contain up to 32 elements or subtrees. This is certainly more expensive than an in-place update in a mutable array, but still a lot cheaper than copying the whole vector.
 
 Because vectors strike a good balance between fast random selections and fast random functional updates, they are currently the default implementation of immutable indexed sequences:
 
-
     scala> collection.immutable.IndexedSeq(1, 2, 3)
     res2: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3)