Expose PNG metadata comments (#4157)

lib/input.js

@@ -450,6 +450,7 @@ function _isStreamInput () {
  * - `xmp`: Buffer containing raw XMP data, if present
  * - `tifftagPhotoshop`: Buffer containing raw TIFFTAG_PHOTOSHOP data, if present
  * - `formatMagick`: String containing format for images loaded via *magick
+ * - `comments` Array of keyword/text pairs representing PNG text blocks, if present.
  *
  * @example
  * const metadata = await sharp(input).metadata();