Is there a reason why xref_comment/[3,4] is not in the docs? There seem to be other predicates of the prolog_xref module not in the docs.
My answer, which may not be correct, is that it is a helper predicate for the Cross-referencer tool. The predicate xref_comment/[3,4] was not created to be used standalone and thus not documented.
If xref_comment/[3,4] is a helper, then why is it exported?
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Good question. Don’t have a good solid answer. 
There must have been some kind of bug that was fixed in the current git version.
xref_comment/[3,4] docs appear in the git version using doc_browser; but if you run doc_browser with 8.1.10 the docs for prolog_xref don’t appear in the index, but they are found if you do a search:
If you do a search:
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Most libraries are documented in the manual. Others, notably old, deprecated or mostly internal-use libraries are not. Their PlDoc comments should show up in the online PlDoc browser and notably on the website as this makes sure the PlDoc comments of all provided libraries is included.
For short, use the website to find these predicates.
looks like a bug. Type-ahead works 
They also don’t show up on the main page for the prolog_xref library (if you click on library(prolog_xref): Cross-reference data collection library.
That probably explains what is wrong. The library is documented using the old LaTeX documentation, but this hasn’t been updated for a long time. This should be replaced by the PlDoc documentation.