rework phrasing

Author: t-bltg

docs/src/index.md

@@ -237,7 +237,7 @@ Alternatively, `render(str, mime)` can also be used to generate and display DVI,
 latexify(:(x/y)) |> s -> render(s, MIME("image/png"))
 ```
 
-PNG output relies on [ghostscript](https://www.ghostscript.com) or alternatively on [dvipng](http://www.nongnu.org/dvipng) (use the `convert` argument).
+PNG output relies on [ghostscript](https://www.ghostscript.com) or alternatively on [dvipng](http://www.nongnu.org/dvipng): if `render` is called with the keyword `convert = :gs`, ghostcript will be used to convert the `pdf` output to `png` format.
 
 SVG output relies on [dvisvgm](https://dvisvgm.de) or alternatively on [pdf2svg](https://github.com/dawbarton/pdf2svg).