The best source of information about Borland's TLINK/Turbo Debugger debugging information is probably the Borland Open Architecture Handbook (available in many places, e.g. here). It describes the updated, 32-bit supporting debug info (see "Symbol table format" section), but a lot of it still applies to 16-bit one.
Ralph Brown also has a short description of the header: http://www.delorie.com/djgpp/doc/rbinter/it/24/16.html