sys/vfs/hammer: Fix and add comments on btree

sys/vfs/hammer: Fix and add comments on btree

- hunk1-2: These two seem to be old ones. At this point *node is
  a pointer to cursor->node->ondisk which is not invalid or stale.

- hunk3: This isn't necessary since now this function just returns
  EINVAL if the cursor's node is internal. It didn't have this check
  when this comment was written.

- hunk4: It's hammer_btree_lookup() that could return ENOSPC when
  no space is available, but not this function. Moving this comment
  to the previous paragraph makes better sense.

- hunk5: Leaf nodes have no idea of boundaries to start with so
  replace it with the comment being used in hammer_btree_delete()
  which also deals with bcopy() of elements.

- hunk6: Fix a typo.

- hunk7: Refer to a leaf node's count in addition to internal node.