Provided docroot in _CoqProject to avoid warnings

_CoqProject

@@ -5,6 +5,8 @@
 # Cannot use non-canonical projections as it causes massive unification failures
 # (https://github.com/coq/coq/issues/6294).
 -arg -w -arg -redundant-canonical-projection
+# No common logical root, so we have to specify documentation root, to avoid warnings
+-docroot actris
 
 actris/utils/llist.v
 actris/utils/compare.v