Delegation is not experimental

Change wording that described user delegation as an experimental
feature.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

docs/breaking_changes.md

@@ -47,5 +47,5 @@ work as you would expect.
 ## Docker Content Trust
 
 Engine older than the current 1.10 cannot pull images from repositories that
-have enabled key delegation. Key delegation is an experimental feature which
-requires a manual action to enable.
+have enabled key delegation. Key delegation is a feature which requires a
+manual action to enable.