doc: surface: add some source code level documentation
doc: surface: add some source code level documentation

Signed-off-by: André Draszik <4a4cb09e7b43b167f95b1dc166b9a7dec9f65d7c@st.com>

lancebaiyouview/DirectFB,lancebaiyouview/DirectFB,kevleyski/DirectFB-1,jcdubois/DirectFB,DirectFB/directfb,dfbdok/DirectFB1,Distrotech/DirectFB,djbclark/directfb-core-DirectFB,kaostao/directfb,DirectFB/directfb,djbclark/directfb-core-DirectFB,deniskropp/DirectFB,jcdubois/DirectFB,jcdubois/DirectFB,djbclark/directfb-core-DirectFB,kevleyski/DirectFB-1,kaostao/directfb,sklnet/DirectFB,deniskropp/DirectFB,kevleyski/DirectFB-1,kaostao/directfb,kevleyski/directfb,mtsekm/test,deniskropp/DirectFB,kevleyski/DirectFB-1,sklnet/DirectFB,lancebaiyouview/DirectFB,dfbdok/DirectFB1,deniskropp/DirectFB,sklnet/DirectFB,kevleyski/directfb,kevleyski/directfb,mtsekm/test,kevleyski/directfb,Distrotech/DirectFB,DirectFB/directfb,dfbdok/DirectFB1,Distrotech/DirectFB,sklnet/DirectFB,mtsekm/test,djbclark/directfb-core-DirectFB,lancebaiyouview/DirectFB