DOC: regtest: make it clearer what the purpose of the "broken" series is

The purpose of the "broken" series of reg tests is to integrate scripts
which are known for triggering bugs that are not fixed at the time the
script is merged. These ones are not useful to validate non-regression
after merging a change, but have an important value to help fix the bug
they trigger. This patch updates the description in the Makefile to make
this clearer.
diff --git a/Makefile b/Makefile
index 934c85b..cc29b1a 100644
--- a/Makefile
+++ b/Makefile
@@ -1129,7 +1129,7 @@
 	@echo "    LEVEL 2 scripts are slow scripts (prefixed with 's' letter)."
 	@echo "    LEVEL 3 scripts are low interest scripts (prefixed with 'l' letter)."
 	@echo "    LEVEL 4 scripts are in relation with bugs they help to reproduce (prefixed with 'b' letter)."
-	@echo "    LEVEL 5 scripts are broken scripts, typically used to fastly disable broken scripts (prefixed with 'k' letter)."
+	@echo "    LEVEL 5 scripts are scripts triggering known broken behaviors for which there is still no fix (prefixed with 'k' letter)."
 	@echo "    LEVEL 6 scripts are experimental, typically used to develop new scripts (prefixed with 'e' lettre)."
 
 .PHONY: reg-tests reg-tests-help