Say what a 0 side does in the ValidateDimension doc comment

It said 0 meant "original size"; the side is worked out from the
source's proportions, as README.md now says.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -29,7 +29,9 @@ var (
 const MaxDimension = imageprocessor.MaxOutputDimension
 
 // ValidateDimension checks one requested width or height; name ("width" or
-// "height") appears in the error. 0 means "original size" and is valid.
+// "height") appears in the error. 0 is valid: that side is then worked out from
+// the source's proportions and the other requested side, rounded down but at
+// least 1, or is the source's own side when the other side is 0 too.
 func ValidateDimension(name string, value int) error {
 	if value < 0 {
 		return fmt.Errorf("%w: %s is negative", ErrInvalidSize, name)