Add docstring to ConfigurableInstance.

Also fix a couple of minor other docstring problems, but not yet conforming to numpydoc standards.

python/lsst/pex/config/configurableField.py

@@ -28,6 +28,13 @@
 
 
 class ConfigurableInstance:
+    """A retargetable configuration for a configurable object.
+
+    Use the `value` property to access the actual Config instance; use the
+    target` property to access the associated configurable object (e.g. a
+    `Task`).
+    """
+
     def __initValue(self, at, label):
         """
         if field.default is an instance of ConfigClass, custom construct
@@ -74,15 +81,14 @@ def __init__(self, config, field, at=None, label="default"):
 
     def apply(self, *args, **kw):
         """
-        Call the confirurable.
+        Call the configurable.
         With argument config=self.value along with any positional and kw args
         """
         return self.target(*args, config=self.value, **kw)
 
-    """
-    Target a new configurable and ConfigClass
-    """
     def retarget(self, target, ConfigClass=None, at=None, label="retarget"):
+        """Target a new configurable and ConfigClass
+        """
         if self._config._frozen:
             raise FieldValidationError(self._field, self._config, "Cannot modify a frozen Config")