Add comment about Newtek NDI times resolution

See: https://github.com/FFmpeg/FFmpeg/blob/master/libavdevice/libndi_newtek_common.h#L27

From NDI SDK Documentation:
This is the timecode of this frame in 100ns intervals. This is generally not used internally by the SDK, but is passed
through to applications who may interpret it as they wish. When sending data, a value of
NDIlib_send_timecode_synthesize can be specified (and should be the default), the operation of this value is
documented in the sending section of this documentation. NDIlib_send_timecode_synthesize will yield UTC
time in 100ns intervals since the Unix Time Epoch 1/1/1970 00:00. When interpreting this timecode a receiving
application may choose to localise the time of day based on time zone offset which can optionally be communicated by
the sender in connection metadata. Since timecode is stored in UTC within NDI, communicating timecode time of day for
non UTC time zones requires a translation

src/ndiaudiosrc.rs

@@ -388,6 +388,7 @@ impl BaseSrcImpl<BaseSrc> for NdiAudioSrc {
 
                 let buffer = buffer.get_mut().unwrap();
 
+                // Newtek NDI yields times in 100ns intervals since the Unix Time
                 let pts: gst::ClockTime = (pts * 100).into();
                 buffer.set_pts(pts + ndi_struct.start_pts);
 

src/ndivideosrc.rs

@@ -386,6 +386,7 @@ impl BaseSrcImpl<BaseSrc> for NdiVideoSrc {
             let mut buffer = gst::Buffer::with_size(buff_size).unwrap();
             {
                 let vec = Vec::from_raw_parts(video_frame.p_data as *mut u8, buff_size, buff_size);
+                // Newtek NDI yields times in 100ns intervals since the Unix Time
                 let pts: gst::ClockTime = (pts * 100).into();
 
                 let duration: gst::ClockTime = (((f64::from(video_frame.frame_rate_D)