expo: Add documentation for the configuration editor

This is mentioned in passing in the 'cedit' command. Its file format is
described under `expo`. But it would be better if it had its own entry
in the documentation.

Add a new 'cedit' entry with a few details about this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/cedit.rst b/doc/usage/cmd/cedit.rst
index 8e1110c..3d815bd 100644
--- a/doc/usage/cmd/cedit.rst
+++ b/doc/usage/cmd/cedit.rst
@@ -22,6 +22,8 @@
 The description is in the form of a devicetree file, as documented at
 :ref:`expo_format`.
 
+See :doc:`../../develop/cedit` for information about the configuration editor.
+
 Example
 -------