core: Fix USB functionfs activation and clarify its documentation (#4188)

core: Fix USB functionfs activation and clarify its documentation (#4188)

There was no certainty about how the path in service file should look
like for usb functionfs activation. Because of this it was treated
differently in different places, which made this feature unusable.

This patch fixes the path to be the *mount directory* of functionfs, not
ep0 file path and clarifies in the documentation that ListenUSBFunction should be
the location of functionfs mount point, not ep0 file itself.