Specified that Images cannot do text substitution

This commit specifies that images cannot substitute a piece of text with
itself. This means that end bounds of those segments are ignored, and
images will be inserted where the start bounds point them to.

Frontends that can't actually display images should represent this
information in another way by itself. One way is to treat this as a
hyperlink that says "Image."

text/text.go

@@ -28,7 +28,7 @@ type Linker interface {
 }
 
 // Imager implies the segment should be replaced with a (possibly inlined)
-// image.
+// image. Only the starting bound matters, as images cannot substitute texts.
 type Imager interface {
 	Segment
 	Image() (url string)