Add note to Store about uniqueness validation (#17954) [skip ci]

activerecord/lib/active_record/store.rb

@@ -15,11 +15,14 @@ module ActiveRecord
   # You can set custom coder to encode/decode your serialized attributes to/from different formats.
   # JSON, YAML, Marshal are supported out of the box. Generally it can be any wrapper that provides +load+ and +dump+.
   #
-  # NOTE - If you are using PostgreSQL specific columns like +hstore+ or +json+ there is no need for
+  # NOTE: If you are using PostgreSQL specific columns like +hstore+ or +json+ there is no need for
   # the serialization provided by +store+. Simply use +store_accessor+ instead to generate
   # the accessor methods. Be aware that these columns use a string keyed hash and do not allow access
   # using a symbol.
   #
+  # NOTE: The default Validations with the exception of +uniqueness+ will work. For example, if you
+  # want to check for +uniqueness+ with +hstore+ you will need to use a custom validation to handle it.
+  #
   # Examples:
   #
   #   class User < ActiveRecord::Base