Improve CustomConfig documentation

Improve documentation about what CustomConfigHelperMixin does,
it was not very clear without a close look to the code before.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.draft.rst

@@ -20,6 +20,7 @@ New Features
 Changes
 -------
 
+* Improve CustomConfig documentation.
 * Imply single threading mode in debug mode exept for "-d thread".
 * Code and import cleanups.
 * Allow UI to have arbitrary names.

offlineimap/CustomConfig.py

@@ -71,18 +71,33 @@ def getsectionlist(self, key):
                 if x.startswith(key)]
 
 def CustomConfigDefault():
-    """Just a sample constant that won't occur anywhere else to use for the
-    default."""
+    """Just a constant that won't occur anywhere else.
+
+    This allows us to differentiate if the user has passed in any
+    default value to the getconf* functions in ConfigHelperMixin
+    derived classes."""
     pass
 
 class ConfigHelperMixin:
+    """Allow comfortable retrieving of config values pertaining to a section.
+
+    If a class inherits from this cls:`ConfigHelperMixin`, it needs
+    to provide 2 functions: meth:`getconfig` (returning a
+    ConfigParser object) and meth:`getsection` (returning a string
+    which represents the section to look up). All calls to getconf*
+    will then return the configuration values for the ConfigParser
+    object in the specific section."""
+
     def _confighelper_runner(self, option, default, defaultfunc, mainfunc):
-        if default != CustomConfigDefault:
-            return apply(defaultfunc, [self.getsection(), option, default])
-        else:
+        """Return config value for getsection()"""
+        if default == CustomConfigDefault:
             return apply(mainfunc, [self.getsection(), option])
+        else:
+            return apply(defaultfunc, [self.getsection(), option, default])
 
-    def getconf(self, option, default = CustomConfigDefault):
+
+    def getconf(self, option,
+                default = CustomConfigDefault):
         return self._confighelper_runner(option, default,
                                          self.getconfig().getdefault,
                                          self.getconfig().get)
@@ -101,4 +116,3 @@ def getconffloat(self, option, default = CustomConfigDefault):
         return self._confighelper_runner(option, default,
                                          self.getconfig().getdefaultfloat,
                                          self.getconfig().getfloat)
-