Add comments, change structure member name.

"GSList *" implies a list; call the pointer in a struct
heur_dissector_list "dissectors", to indicate what it's a list *of*.

Change-Id: Idf5011e08d35d28934cd52f818b945db1cc7f37b
Reviewed-on: https://code.wireshark.org/review/5941
Reviewed-by: Guy Harris <guy@alum.mit.edu>