Update user guide and scheduler documentation to describe node affinity.

Register image priority locality function, which the original PR that introduced
it forgot to do. Change zone and region labels to beta.

docs/user-guide/labels.md

@@ -48,6 +48,7 @@ Documentation for other releases can be found at
     - [Set references in API objects](#set-references-in-api-objects)
       - [Service and ReplicationController](#service-and-replicationcontroller)
       - [Job and other new resources](#job-and-other-new-resources)
+      - [Selecting sets of nodes](#selecting-sets-of-nodes)
 
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@@ -211,6 +212,11 @@ selector:
 
 `matchLabels` is a map of `{key,value}` pairs. A single `{key,value}` in the `matchLabels` map is equivalent to an element of `matchExpressions`, whose `key` field is "key", the `operator` is "In", and the `values` array contains only "value". `matchExpressions` is a list of pod selector requirements. Valid operators include In, NotIn, Exists, and DoesNotExist. The values set must be non-empty in the case of In and NotIn. All of the requirements, from both `matchLabels` and `matchExpressions` are ANDed together -- they must all be satisfied in order to match.
 
+#### Selecting sets of nodes
+
+One use case for selecting over labels is to constrain the set of nodes onto which a pod can schedule.
+See the documentation on [node selection](node-selection/README.md) for more information.
+
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