add doc comments

src/backend/loadRawImage.mli

@@ -1,3 +1,17 @@
+(**
+   A Generic image loader for PDF.
+*)
+
 open MyUtil
 
+(**
+   [make_xobject pdfmain colorspace widdots hgtdots abs_path] creates a PDF XObject Image
+   for the file located at [abspath] with ColorSpace = [colorspace], Width = [widdots], and Height = [hgtdots],
+   and then add it to the PDF document [pdfmain].
+   [colorspace] is can be acquired by {!val:ImageHashTable.find}.
+   [widdots] and [hgtdots] are the image width and height in dots.
+
+   This function raises {!val:ImageHashTable.CannotLoadImage} if the images format is not supported
+   or the image cannot be loaded due to any other reason.
+*)
 val make_xobject : Pdf.t -> Pdf.pdfobject -> int -> int -> abs_path -> Pdf.pdfobject