hub: priority policy

Fixes: https://projects.engineering.redhat.com/browse/RHELBLD-4275

docs/source/defining_hub_policies.rst

@@ -10,6 +10,7 @@ in the system. At present, policy allows you to control:
 * allowing builds from expired repos
 * managing the package list for a tag
 * managing which channel a task goes to
+* altering task priority
 
 In the future, we expect to add more policy hooks for controlling more aspects
 of the system.
@@ -161,6 +162,28 @@ the following actions:
     * only valid for child tasks
     * recommend using the ``is_child_task`` test to be sure
 
+The priority policy is used to alter task's priority. In most cases you should
+manage priorities by different channels and builders assigned to them. There is
+nevertheless few corner-cases which can benefit from altering task's priority.
+Note, that you can easily get to deadlock situation if this is not handled with
+caution (lower priority tasks will get assigned only if there is no higher
+priority task for given channel).
+
+Technically it is very similar to ``channel`` policy. Only actions are
+different:
+
+``stay``
+    * don't touch the default priority of the task
+
+``set <int>``
+    * set priority to this value
+
+``increment by <int>``
+    * increment default priority
+
+``decrement by <int>``
+    * decrement default priority
+
 Available tests
 ===============
 ``true``