Doc: --library should mention .lock
The doumentation for command-line parameter --library should mention that next to the library, a .lock file is created, to aid in resetting something gone awry:
Whenever Darktable starts, it will lock the library to the current user. It does this by writing the current process identifier (PID) into a lock file (<mylibrary>.lock) next to the library specified.
If Darktable finds an existing lock file for the library, it will terminate immediately.
Without this information, it is not easy to recover from an aborted Darktable process.
This is not exactly the best place to introduce the concept of what is, in a way, "single process per user" behaviour (is this documented at all?), but for 99% of the population it will be the right location; the other 1% will understand.