Signal does not provide any documentation for command line options, the problem is already discussed on GitHub, since 2019!
This is a brief compilation of known options and will update if new ones are discovered until Signal provides official documentation.
--enable-dev-tools
Enables Developer Tools sidebar (Ctrl+Shift+I) and adds a shortcut to Menu Bar to open them.
--enable-gpu
Enables GPU acceleration.
--use-tray-icon
Enables Signal to display an icon in the system tray and allows for minimizing to the tray instead of exiting the app. How to.
--user-data-dir
Facilitates running another instance of the client with separate user data directories, useful for managing multiple users or numbers to log in with.
--start-in-tray
Initiates Signal with the window minimized to the system tray.
signal-desktop --user-data-dir=$HOME/.config/ProfileName
Other options
These options are in the Signal Desktop source code, but I haven’t managed to check whether they work and what they actually do.