Gets or sets a value indicating whether, in structure order, the alternate description (Alt) of a structure element
replaces the text of the glyphs it covers, as a whole word or phrase, flagged AlternateDescription.
Default false: the text is what the page shows; ActualText, a replacement for what a person sees,
always applies.
public bool ApplyAlternateDescriptions { get; set; }
ISO 32000-2 §14.9.3 and §14.8.2.2.1 (a processor may substitute an element with its alternate description).