udpated upgrade doc

docs/upgrading_to_v4.0.md

@@ -3,7 +3,7 @@
 The v4.0 release contains backwards-incompatible changes.
 
 ### TPG max version is bumped to 6.10 for regional-backend-security-policy module
-There is no known breaking change for Cloud Armor in 6.X.
+TPG 6.10 added support for overriding default security rule
 
 ### Added default rule at priority 2147483647
-Before this version a default security rule with priority 2147483647 was created. This update will override that rule so users can manage it in terraform
+Before this version a default security rule with priority 2147483647 was created behind the scene but was not part of TF state file. This update will override that rule so users can manage it in terraform