docs: add section about debug commands

docs/usage.rst

@@ -402,6 +402,16 @@ Miscellaneous Help
 .. include:: usage/help.rst.inc
 
 
+Debug Commands
+--------------
+There are some more commands (all starting with "debug-") wich are are all
+**not intended for normal use** and **potentially very dangerous** if used incorrectly.
+
+They exist to improve debugging capabilities without direct system access, e.g.
+in case you ever run into some severe malfunction. Use them only if you know
+what you are doing or if a trusted |project_name| developer tells you what to do.
+
+
 Additional Notes
 ----------------