Improve documentation for display property functions

Improve documentation for display property functions

Specifically, use the term "display specification" more consistently to
distinguish from "display property", which is the full value of the
'display' text property.

* src/xdisp.c (find_display_property): Rename PROP to SPEC.
(Fget_display_property): Rename PROP to SPEC and improve docstring.

* lisp/emacs-lisp/subr-x.el (add-display-text-property): Rename PROP to
SPEC and improve docstring.

* doc/lispref/display.texi (Display Property): Reword documentation to
more-consistently refer to display specifications.

(cherry picked from commit 24e6cd42330c341f3525e3fdc384e4a646dec733)