Update Set doc

lib/concurrent/set.rb

@@ -16,9 +16,12 @@ module Concurrent
     #
     #   @note `a += b` is **not** a **thread-safe** operation on
     #   `Concurrent::Set`. It reads Set `a`, then it creates new `Concurrent::Set`
-    #   which is concatenation of `a` and `b`, then it writes the concatenation to `a`.
+    #   which is union of `a` and `b`, then it writes the union to `a`.
     #   The read and write are independent operations they do not form a single atomic
     #   operation therefore when two `+=` operations are executed concurrently updates
+    #   may be lost. Use `#merge` instead.
+    #
+    #   @see http://ruby-doc.org/stdlib-2.4.0/libdoc/set/rdoc/Set.html Ruby standard library `Set`
     class Set < ::Set;
     end