Clarify xreflabel inheritance and examples

[graphitefriction]

The xreflabel needs its own subsection and specific examples showing the alternate ways to customize it. Refer to #380

[mojavelinux]

Let's be sure to explain the search priority. When determining the label to put at the location of the cross reference, it searches for text in the following order:

1. The explicit text specified on the cross reference itself
2. The value of the reftext on the object to which the cross reference points
3. The title of the object to which the cross reference points

(It may be useful to note that DocBook calls the reftext the xreflabel. They are one and the same).