IR: Cleanup comments for Value, User, and MDNode

A follow-up commit will modify the memory-layout of `Value`, `User`, and
`MDNode`.  First fix the comments to be doxygen-friendly (and to follow
the coding standards).

  - Use "\brief" instead of "repeatedName -".
  - Add a brief intro where it was missing.
  - Remove duplicated comments from source files (and a couple of
    noisy/trivial comments altogether).