; Improve documentation of image properties

; Improve documentation of image properties

* lisp/image.el (create-image, image-property): Add to do strings
link to description of image properties in ELisp manual.

* doc/lispref/display.texi (Defining Images): Fix example and add
cross-reference to where image properties are described.
(Image Descriptors): Add index entry.

(cherry picked from commit 5ffcca121bb79b97c6a0f941c71a61505032d8f8)