Clarify where multiple inline anchors are supported and their purpose

[mojavelinux]

Section titles support multiple anchors. Other blocks do not, such as a list item or description list term. The purpose of multiple anchors is to preserve references from outside the document. They add a fragment identifier to the node so that it can be the target of a deep link. They do no work for defining xrefs with automatic text within the document. The user manual should frame this topic accurate to set expectations.