Update for wpi::Now() change to microseconds. (#113)

Also be explicit in docs about what timebase and step are used.

src/main/native/include/cscore_oo.h

@@ -122,6 +122,8 @@ class VideoSource {
   std::string GetDescription() const;
 
   /// Get the last time a frame was captured.
+  /// This uses the same time base as wpi::Now().
+  /// @return Time in 1 us increments.
   uint64_t GetLastFrameTime() const;
 
   /// Is the source currently connected to whatever is providing the images?
@@ -578,13 +580,15 @@ class CvSink : public VideoSink {
   /// Times out (returning 0) after timeout seconds.
   /// The provided image will have three 8-bit channels stored in BGR order.
   /// @return Frame time, or 0 on error (call GetError() to obtain the error
-  ///         message);
+  ///         message); the frame time is in the same time base as wpi::Now(),
+  ///         and is in 1 us increments.
   uint64_t GrabFrame(cv::Mat& image, double timeout = 0.225) const;
 
   /// Wait for the next frame and get the image.  May block forever.
   /// The provided image will have three 8-bit channels stored in BGR order.
   /// @return Frame time, or 0 on error (call GetError() to obtain the error
-  ///         message);
+  ///         message); the frame time is in the same time base as wpi::Now(),
+  ///         and is in 1 us increments.
   uint64_t GrabFrameNoTimeout(cv::Mat& image) const;
 
   /// Get error string.  Call this if WaitForFrame() returns 0 to determine