Hi. With a recent xmllint (using libxml version 21500-GITv2.15.0), doc compilation fails with:
/usr/bin/xmllint --nonet --path . --path . --output postgres-full.xml --noent --valid postgres.sgml
ref/pg_combinebackup.sgml:317: validity error : standalone: normalization of attribute linkend on xref by external
subsetdeclaration
state of the cluster using <xref linkend="app-pgchecksums "/> is
^
Attached a patch to solve the issue.