PNG: Document the "png:compressionLevel" output hint. (#2642)
PNG: Document the "png:compressionLevel" output hint. (#2642)

Signed-off-by: Larry Gritz <f0a1ce6f80112d72f7276457fd261d671b827567@larrygritz.com>
OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio