Clarifications and overview of functionality

"If anyone can shed light on what some of the more mysterious parameters do please let me know!"
You can ~~steal~~ copy examples from [here](https://www.freedesktop.org/wiki/Software/systemd/logind/)

[Overview of functionality](https://www.commandlinux.com/man-page/man8/systemd-logind.service.8.html)

"Keeping track of users and sessions, their processes and their idle state" and "shutdown/sleep inhibition logic for applications" is missing from the big pictures, which could link as table of contents.

It would be probably easier to just copy-paste [these images](https://dvdhrm.wordpress.com/2013/08/25/sane-session-switching/) and remove/add an appendix section for `libinput` and `udev`, since `logind` does only do the file-descriptor handling.

"This could probably be done by using the posix sessionids in most cases. But those are not reliable since processes can leave their session and make a new one. Now we don't know anymore what is going on!"
Examples for cases would help here

Writing udev functionality as a list ( defining set of hardware as `Seat`, ... ) would certainly help readability.

Review the existing documentation alongside the linked systemd-logind overview and examples. Identify where the overview of functionality, session-ID examples, and udev hardware/Seat explanation belong, including the proposed libinput and udev appendix. Done means the documentation clearly covers these topics and provides useful examples or navigation.

Written by the indexing model from the issue text.