State that comma separated data is serialised data

Comma separated values really are a form of serialised data so we should
clarify that we shouldn't store such data in the DB.

doc/development/serializing_data.md

 # Serializing Data
 
 **Summary:** don't store serialized data in the database, use separate columns
-and/or tables instead.
+and/or tables instead. This includes storing of comma separated values as a
+string.
 
 Rails makes it possible to store serialized data in JSON, YAML or other formats.
 Such a field can be defined as follows: