gpiod.h: improve the documentation for gpiod_line_iter_last_offset()
gpiod.h: improve the documentation for gpiod_line_iter_last_offset()

Mention the undefined behavior if gpiod_line_iter_last_offset() is
ever called before gpiod_line_iter_next().

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod