The PDF writer writes a spot gradient as a Separation or DeviceN shading
only when it is linear or radial; freeform gradients and dithered fills
are written as images in process colors.
- `paint.editGradient`'s method help and the Method paragraph in
docs/mcp.md limit the Perceptual exception to linear and radial
gradients.
- The Linked gradient stops paragraph in docs/mcp.md no longer says the
writer has no DeviceN: a gradient mixing spot inks with other spot,
CMYK or gray inks exports as a DeviceN shading, and only a mix with RGB
or Lab colors is written in process colors, with a warning.