Add some missing documentation, basically

This merge request will:

• Document the following source files:
  • clargs.lua
  • imagefont.lua
  • mapfunc.lua
  • music.lua
• Document these buttons and button groups:
  • DB.LOAD
  • DBS.SAVECANCEL
  • DBS.LOADCANCEL
• Document these level editor variables:
  • editingroomtext
  • editingroomname
  • editingbounds
  • eraserlocked
  • showepbounds
  • tilespicker and tilespicker_shortcut
• Document DF.FILES