small cleanup (#2267)

* session_info: fix authorities docstring

* overseer: more consistent metrics naming

* session_info: mention ordering

* use correct bucket sizes

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>