Add error-message, error-for, details, and details-for relation types
Add error-message, error-for, details, and details-for relation types

Also update documentation of description-for and described-by relation
types to help clarify which relation type is most suitable for each of
the accessible descriptions.

https://bugzilla.gnome.org/show_bug.cgi?id=781587

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core