docs(compat): Add missing compat admonition to IdSet docstring (#59290)

Author: kunzaatko

base/idset.jl

@@ -10,6 +10,9 @@
 In the example below, the values are all `isequal` so they get overwritten in the ordinary `Set`.
 The `IdSet` compares by `===` and so preserves the 3 different values.
 
+!!! compat "Julia 1.11"
+    Exported in Julia 1.11 and later.
+
 # Examples
 ```jldoctest; filter = r"\\n\\s*(1|1\\.0|true)"
 julia> Set(Any[true, 1, 1.0])