Address DG comments made by TA and peers

[xenthm]

• In DG UML diagrams, give participants boxes a style in the config files such that the corners are not rounded
• In AuthorListClass.puml:
  Perhaps the UML note can be connected by a dotted line instead!
  
  Originally posted by @chewycj in [CS2113-T10-3] MangaTantou nus-cs2113-AY2425S1/tp#9 (comment)
• In ViewMangaSequence.puml:
  
  I think you should remove the "done printing" label, as the dotted arrow is sufficient to indicate that control has gone back to Tantou?
  Originally posted by @jemehgoh in [CS2113-T10-3] MangaTantou nus-cs2113-AY2425S1/tp#9 (comment)
• In refGetColumnsToPrintSequence.puml:
  
  I think you don't have to mention "added to columns" every time you return control, since you already mention "add new [column]" for each corresponding operation?
  Originally posted by @jemehgoh in [CS2113-T10-3] MangaTantou nus-cs2113-AY2425S1/tp#9 (comment)
• Update ref frames to this format: