doc: add descriptions for doxygen sub-sections
doc: add descriptions for doxygen sub-sections

Add additional descriptions fox doxygen sub-sections under the line
section.

Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

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