They are the ambient light and proximity sensors. The ambient light sensor adjusts the screens brightness according to the ambient lighting conditions. The proximity sensor locks the screen during a phone call when the phone is placed to your ear.
what are these two little things on Lumia 710 front top left corner, and why it's not written about it anywhere in user guide
Depends upon what source you look at:http://teamatlas.co.uk/Nokia%20Lumia710.pdf