doc improvements at package Plumbing

[Serabe]

• Difftree package: document.
• Format:
  • Config:
    • Document Comment
    • Document Config
      • Document undocumented methods
    • Document Include (reword)
    • Document Options:
      • Fix format for Get method.
    • Document Section and its methods.
    • Document Subsection and its methods.
  • idxfile:
    • Reword Decoder.Decode given multiple uses of idx to refer to different things. Same for Encoder.Encode.
    • Document EntryList methods
    • Document Idxfile.Add methods
  • Document index:
    • Document ResolveUndo in a way it makes sense to have the name of the type at the beginning.
  • Document objfile:
    • Document package.
  • Document packfile:
    • Reword GetDelta
    • Clarify DecodeObjectAt. Why if offsets are a map from hashes to ints I am required to set them before reading something using an int?
    • NewEncoder does no document what the third argument is for.
  • Package pktline:
    • In Encodef, what is the arguments suggests?
• Object
  • Reword WalkCommitHistory. Specify what it does, what the inputs are for, what happens if the callback returns an error, etc.
  • Document DecodeBlob.
  • NewBlobIter fix arguments explanation
  • Document Commit fields.
  • In NewObjectIter, rephrase input.
  • In Signature, what is the field that holds the action that this type represents.
• Package protocol
  • Capabilities:
    • Document package
• Package revlist:
  • Documentation says that package implements functions in plural. Only one can be found.
• Package storer:
  • ReferenceStorer type doc update so it reads naturally.
  • ShallowStorer type doc update so it reads naturally.
• Package transport:
  • Package client:
    • Document package.
  • Package file:
    • Document package.
  • Package git:
    • Document package.
  • Package ssh:
    • Document package.