Restore accidentally removed docstring for get_columns method

Author: WaVEV

django_mongodb/compiler.py

@@ -395,6 +395,10 @@ def build_query(self, columns=None):
         return query
 
     def get_columns(self):
+        """
+        Return a tuple of (name, expression) with the columns and annotations
+        which should be loaded by the query
+        """
         select_mask = self.query.get_select_mask()
         columns = (
             self.get_default_columns(select_mask) if self.query.default_cols else self.query.select