improve Active Record Store docs when using PG types. [ci skip].

activerecord/lib/active_record/store.rb

@@ -15,9 +15,10 @@ 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 special PostgreSQL columns like +hstore+ or +json+ there is no need for
-  # the serialization provided by +store+. You can simply use +store_accessor+ instead to generate
-  # the accessor methods.
+  # 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.
   #
   # Examples:
   #