doc: update the descriptions for chip iterators
doc: update the descriptions for chip iterators

The 'next' routines for chip iterators can no longer fail. Update
the docs accordingly. Also: turn the extended description into a note
for the noclose variant.

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

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