[meld] dirdiff: Add some better docstring for our state mutation

[Kai Willadsen <kaiw src gnome org>]

commit 483b4d6bd13551ae7b6cbc3c452dff762a9deaa1
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Fri Dec 16 07:15:15 2016 +1000

    dirdiff: Add some better docstring for our state mutation

 meld/dirdiff.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/meld/dirdiff.py b/meld/dirdiff.py
index 11130f7c..a3c7b759 100644
--- a/meld/dirdiff.py
+++ b/meld/dirdiff.py
@@ -1251,7 +1251,12 @@ class DirDiff(melddoc.MeldDoc, gnomeglade.Component):
         return ret
 
     def _update_item_state(self, it):
-        """Update the state of the item at 'it'
+        """Update the state of a tree row
+
+        All changes and updates to tree rows should happen here;
+        structural changes happen elsewhere, but they only delete rows
+        or add new rows with path information. This function is the
+        only place where row details are changed.
         """
         files = self.model.value_paths(it)
         regexes = [f.byte_filter for f in self.text_filters if f.active]