binman: Document ExpandEntries() in the base class

Move the documentation to the base method as it is with other methods.
Also update it a little while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index d58a730..507760e 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -211,6 +211,17 @@
         return {}
 
     def ExpandEntries(self):
+        """Expand out entries which produce other entries
+
+        Some entries generate subnodes automatically, from which sub-entries
+        are then created. This method allows those to be added to the binman
+        definition for the current image. An entry which implements this method
+        should call state.AddSubnode() to add a subnode and can add properties
+        with state.AddString(), etc.
+
+        An example is 'files', which produces a section containing a list of
+        files.
+        """
         pass
 
     def AddMissingProperties(self, have_image_pos):