clarify key bindings configuration
Currently the docs say (at https://docs.xfce.org/apps/mousepad/start#keybindings )
After its first use, Mousepad will generate the file ~/.config/Mousepad/accels.scm, containing the list of configurable keybindings. To customize a keybinding, uncomment the corresponding line by removing the semicolon at the beginning, then change the shortcut at the end of the line, without touching the rest.
The docs should warn that the file cannot be edited with mousepad itself, nor while mousepad is open.