Add comments to error enums.

Author: Simon Gladman

stdlib/public/Darwin/Accelerate/vImage_Error.swift

@@ -21,25 +21,72 @@ extension vImage {
 
     /// Error codes returned by vImage operations.
     public enum Error: Int, Swift.Error {
+        /// The vImage function completed without error.
         case noError                      =    0
+        
+        // The region of interest, as specified by the `srcOffsetToROI_X` and
+        // `srcOffsetToROI_Y` parameters and the height and width of the destination
+        // buffer, extends beyond the bottom edge or right edge of the source buffer.
         case roiLargerThanInputBuffer     =    -21766
+        
+        /// Either the kernel height, the kernel width, or both, are even.
         case invalidKernelSize            =    -21767
+        
+        /// The edge style specified is invalid.
         case invalidEdgeStyle             =    -21768
+        
+        /// The `srcOffsetToROI_X` parameter that specifies the left edge of
+        /// the region of interest is greater than the width of the source image.
         case invalidOffset_X              =    -21769
+        
+        /// The `srcOffsetToROI_X` parameter that specifies the left edge of
+        /// the region of interest is greater than the height of the source image.
         case invalidOffset_Y              =    -21770
+        
+        /// An attempt to allocate memory failed.
         case memoryAllocationError        =    -21771
+        
+        /// A pointer parameter is NULL and it must not be.
         case nullPointerArgument          =    -21772
+        
+        /// Invalid parameter.
         case invalidParameter             =    -21773
+        
+        /// The function requires the source and destination buffers to have
+        /// the same height and the same width, but they do not.
         case bufferSizeMismatch           =    -21774
+        
+        /// The flag is not recognized.
         case unknownFlagsBit              =    -21775
+        
+        /// A serious error occured inside vImage, which prevented vImage
+        /// from continuing.
         case internalError                =    -21776
+        
+        /// The vImage_Buffer.rowBytes field is invalid.
         case invalidRowBytes              =    -21777
+        
+        /// a `vImage_CGImageFormat` or `vImageCVImageFormatRef` contains
+        /// an invalid format.
         case invalidImageFormat           =    -21778
+        
+        /// ColorSync.framework is completely missing.
         case colorSyncIsAbsent            =    -21779
+        
+        /// The source images and destination images may not alias the same image data.
         case outOfPlaceOperationRequired  =    -21780
+        
+        /// An invalid `CGImageRef` or `CVPixelBufferRef` was passed to the function.
         case invalidImageObject           =    -21781
+        
+        /// A `vImageCVImageFormatRef` contains an invalid format.
         case invalidCVImageFormat         =    -21782
+        
+        /// Some lower level conversion APIs only support conversion among a
+        /// sparse matrix of image formats.
         case unsupportedConversion        =    -21783
+        
+        /// Core Video is absent.
         case coreVideoIsAbsent            =    -21784
 
         public init(vImageError: vImage_Error) {