Add documentation to encoder/decoder fields

[bschwind]

As an example:

nv-video-codec-rs/src/nvdecoder/builder.rs

Lines 9 to 18 in f7f5163

	context: Context,
	use_device_frame: bool,
	codec: Codec,
	low_latency: bool,
	device_frame_pitched: bool,
	crop_rect: Rect,
	resize_dim: Dim,
	max_width: u32,
	max_height: u32,
	clock_rate: u32,

I don't know what device_frame_pitched means, we should get the explanation for this from the nvidia SDK and add inline doc comments on the fields.