Add auto_goal to the docs

Author: basnijholt

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

@@ -6,7 +6,14 @@ 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.
+stopping conditions as a convenience. The `adaptive.runner.auto_goal` will
+automatically create a goal based on simple input types, e.g., an int means
+at least that many points are required and a float means that the loss has
+to become lower or equal to that float.
+
+```{eval-rst}
+.. autofunction:: adaptive.runner.auto_goal
+```
 
 ```{eval-rst}
 .. autofunction:: adaptive.runner.stop_after