doc: add warning that design notes may be inaccurate

Author: xdelaruelle

doc/source/design.rst

@@ -5,6 +5,13 @@ Design notes
 
 Developer notes on feature specifications and code design.
 
+.. warning:: The following notes have been written when the corresponding
+   feature was being developed. Even if special care is given to update these
+   notes later on, their content may not be in line with the current
+   implementation details and behaviors. Please refer to the :ref:`module(1)`
+   and :ref:`modulefile(5)` man pages to learn about current features and
+   options.
+
 ..  toctree::
     :maxdepth: 1
     :glob: