I guess it is not filming any more, since we don't use film.
Most digital cameras do an OK job with video, but not as good as a Camcorder.
The biggest shortcoming is audio.
The digital cameras use a very inexpensive microphone and a very basic audio circuits.
The audio circuit is mainly just automatic gain control.
Very loud sounds will distort badly.
So you will want to look for a digital camera that can accept an external microphone. (there are not many)
You can start by checking which of the cameras you prefer, have an external microphone socket.
which camera you recommend to choose for documentary filming?
I'm not willing to buy camcorder, rather would like to get a photo-camera with HD-video capture capability.
Right now I have in mind Canon 1100D, Canon S100 and Sony DSC-HX20.