Add clarifying comments to xref_spec.rb
Add clarifying comments to xref_spec.rb

In #238, it was discovered that Asciidoctor.load_file + convert is not
equivalent to Asciidoctor.convert_file.

The former:
1. Doesn't write anything to FS
2. Only runs converter on spine document

Given that conversion process mutates document object, it is unsafe and
unpredictable to call conversion multiple times on the same document.

asciidoctor/asciidoctor-epub3