Update documentation to new "COLOR_" format for color conversion enums.

After building from repository, tried following the guide at: http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html#videocapture and got error about CV_BGR2GRAY not defined.  I learned this code was recently changed to COLOR_BGR2GRAY.  I've gone through the .rst files in the master repository and tried to find and replace all occurrences of the old code format "CV_" with the new "COLOR_" format.  This is my first pull request and first time using git, so if I've done something wrong please educate me.  Thanks.