display text property (or overlay property) is used to
insert images into text, and to control other aspects of how text
displays. The value of the
display property should be a
display specification, or a list or vector containing several display
specifications. Display specifications in the same
property value generally apply in parallel to the text they cover.
If several sources (overlays and/or a text property) specify values
display property, only one of the values takes effect,
following the rules of
get-char-property. See Examining Properties.
The rest of this section describes several kinds of display specifications and what they mean.