Documented more changes

--HG--
branch : trunk
diff --git a/jinja2/environment.py b/jinja2/environment.py
index 803e7a8..f163ab5 100644
--- a/jinja2/environment.py
+++ b/jinja2/environment.py
@@ -194,6 +194,7 @@
     #: must not be modified
     shared = False
 
+    #: these are currently EXPERIMENTAL undocumented features.
     exception_handler = None
     exception_formatter = None
 
@@ -475,13 +476,21 @@
         global _make_traceback
         if exc_info is None:
             exc_info = sys.exc_info()
+
+        # the debugging module is imported when it's used for the first time.
+        # we're doing a lot of stuff there and for applications that do not
+        # get any exceptions in template rendering there is no need to load
+        # all of that.
         if _make_traceback is None:
             from jinja2.debug import make_traceback as _make_traceback
+
         traceback = _make_traceback(exc_info, source_hint)
+
         if rendered and self.exception_formatter is not None:
             return self.exception_formatter(traceback)
         if self.exception_handler is not None:
             self.exception_handler(traceback)
+
         exc_type, exc_value, tb = traceback.standard_exc_info
         raise exc_type, exc_value, tb