simple-api: improve documentation
simple-api: improve documentation

Mention the meaning of a positive value returned from the polling
callback of the simple event loop routines.

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

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