Note that you shouldn’t care. QADBIFLD isn’t really for your consumption and you possibly should act as if it doesn’t exist.
What you want to pay attention to are the VIEWs that are built over QADBIFLD (and over similar DDS PFs in QSYS). In your specific case, the VIEW that you’d be interested in would probably be QSYS2/SYSCOLUMNS.
The reason that might make sense to you is because (1) SYSCOLUMNS is obviously a SQL object and (2) you can affect the related QADBIFLD field through the SQL COMMENT ON statement.
Use STRSQL or other interface to run COMMENT ON statements. Reference your files and fields in the COMMENT ON statement. QADBIFLD will be updated for you automatically with your comments.