Compat annotation for #29679 (isnothing).

Author: fredrikekre

base/some.jl

@@ -45,6 +45,9 @@ notnothing(::Nothing) = throw(ArgumentError("nothing passed to notnothing"))
     isnothing(x)
 
 Return `true` if `x === nothing`, and return `false` if not.
+
+!!! compat "Julia 1.1"
+    This function requires at least Julia 1.1.
 """
 isnothing(::Any) = false
 isnothing(::Nothing) = true