Fix #8015: update docs for vararg patterns

Author: liufengyun

docs/docs/reference/changed-features/vararg-patterns.md

@@ -36,5 +36,8 @@ The change to the grammar is:
 
 ## Compatibility considerations
 
-Under the `-language:Scala2Compat` option, Dotty will accept both the old and the new syntax.
-A migration warning will be emitted when the old syntax is encountered.
+To enable smooth cross compilation between Scala 2 and Scala 3, Dotty will
+accept both the old and the new syntax. Under the `-strict` setting, an error
+will be emitted when the old syntax is encountered. They will be enabled by
+default in version 3.1 of the language.
+