add stopping critera documentation to the 'runner.extras' page

docs/source/reference/adaptive.runner.extras.rst

@@ -1,6 +1,17 @@
 Runner extras
 =============
 
+Stopping Criteria
+-----------------
+
+Runners allow you to specify the stopping criterion by providing
+a ``goal`` as a function that takes the learner and returns a boolean: ``False``
+for "continue running" and ``True`` for "stop". This gives you a lot of flexibility
+for defining your own stopping conditions, however we also provide some common
+stopping conditions as a convenience.
+
+.. autofunction:: adaptive.runner.stop_after
+
 Simple executor
 ---------------