sys/vfs/hammer: Fix and add comments on root inode

sys/vfs/hammer: Fix and add comments on root inode

- hunk1 - The root inode uses HAMMER_DEF_LOCALIZATION for its
  onmemory ip(inode) localization, however the purpose of this macro
  is not documented anywhere. Make it clear by adding this comment.

- hunk2 - Show HAMMER_OBJID_ROOT is the root inode # for both real
  root inode and PFS root inodes.

- hunk3-4 - Change "stored in the root inode" to "associated with
  the root inode". PFS records are not embedded within the ondisk
  inode structure, but associated with the root inode by id.
  Commit ea434b6f uses "associated with" in its commit message.