Improve the description texts of the "Install addons" workflow

[tsterbak]

The instructions for the "Install addons" workflow are pretty unclear and not intuitive.

Two things to add:

• explicitly describe how to select multiple addons for flashing. (select all the files you want to flash at once in the filepicker)
• Mention, that before the flashing of the addons starts, you have to activate sideloading in twrp. (tap 'advanced' in TWRP and then 'adb sideload').
• clarify the question "recovery already installed?" at the beginning. (something like "if you didn't do anything or don't know what this means, you most likely do not need to check here)