Fix image constructor documentation

[aurelienpascal]

According the links below, the storage type and pixel map need an update for the Image constructor: https://github.com/ImageMagick/ImageMagick/blob/2a69677d2e41292f23938073c3a2b78380f7d5ed/MagickCore/constitute.c#L201 https://github.com/ImageMagick/ImageMagick/blob/2a69677d2e41292f23938073c3a2b78380f7d5ed/MagickCore/pixel.c#L4197 https://github.com/ImageMagick/ImageMagick/blob/2a69677d2e41292f23938073c3a2b78380f7d5ed/MagickCore/pixel.h#L154