Warning about issue numbers.

doc/update/6.0-to-6.1.md

 # From 6.0 to 6.1
-# In 6.1 we removed a lot of deprecated stuff.
-# So you should update to 6.0 before continue
+# In 6.1 we remove a lot of deprecated code.
+# You should update to 6.0 before installing 6.1 so all the necessary conversions are run.
+
+### Deprecations
+
+#### Global issue numbers
+
+In 6.1 issue numbers are project specific. This means all issues are renumbered and get a new number in their url. If you use an old issue number url and the issue number does not exist yet you are redirected to the new one. This conversion does not trigger if the old number already exists for this project, this is unlikely but will happen with old issues and large projects.
 ### 0. Backup