Documents most of the bar commands in sway-bar(5) manpage.
The following command has not been document because they haven't been
fully implemented yet:
* mode
* hidden_state
* modifier
* tray_output
* tray_padding
Close#375
In anticipation for #375, reorganized and augmented slightly sway(5)
so it makes a difference between commands intended for configuration,
commands intended for control, and those that can serve as both.