Remove unused variable `relative-line-numbers--timer`.
Remove unneeded kill-buffer-hook.
Update eagerly when the delay is set to 0.
Rename `--set-buffer-margin` to `--set-current-buffer-margin`.
Fix handling of multiple windows showing the same buffer.
Add rudimentary docstrings to some internal functions.
Extract the similar loops in relative-line-numbers--update-window into a macro.
Actually fix handling of multiple windows showing the same buffer.