Compat annotation for #29623 (Event).

Author: fredrikekre

base/locks.jl

@@ -246,6 +246,9 @@ Create a level-triggered event source. Tasks that call [`wait`](@ref) on an
 `Event` are suspended and queued until `notify` is called on the `Event`.
 After `notify` is called, the `Event` remains in a signaled state and
 tasks will no longer block when waiting for it.
+
+!!! compat "Julia 1.1"
+    This functionality requires at least Julia 1.1.
 """
 mutable struct Event
     lock::Mutex