This brings the repmgr documentation build system in line with that
used by the main PostgreSQL project, and removed the restriction
that documentation must be built against PostgreSQL 9.6 or earlier.
Main formatting changes are:
- convert empty-element tags (mainly <xref/>)
- put <indexterm> sections in the correct location
- correct usage of various entities.
We only want to check the status of physical replication slots
to determine whether a streaming replication standby has become
detached and there is therefore a risk of uncontrolled WAL buildup
on the local node.
It's not feasible to second-guess the state of logical replication
slots.
