Part of the reason that there's no full list in documentation is that the list is dependent on your implementation of OTRS.
The second part is that it's actually telling you what the * is, as completely as it can.
It won't have all the fields available to you because it doesn't know what you know.
For instance, if you don't have ITSM installed, there's no use telling you what ITSM fields you can use because they don't exist for you.
If you've customized your Map in Config.pm, the fields you've defined/implemented will be relevant.
If you haven't, then the values in the Map in Defaults.pm might be useful.
Comment isn't necessarily listed herein, but is obtained from the User Preferences. Some values aren't really worth presenting to the customer in an email, but may also be stored in the database.
You might be able to GetPreferences on a user to obtain more fields in use.
Dynamic Fields are also custom to your implementation.
If you're looking for something specific that wasn't addressed in this or the sample provided in the interface, I'm sure there's someone who can assist on this list. But there really isn't any master list. Just a list of what is likely functional in the default fields for what one might include in an email.