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.
Default was previously NOTICE (as in repmgr 3.x) but documentation
implied it was INFO, and many of the the documentation examples assume
it is.
This produces some quite informative log output, without creating excessive
log file volume. In particular it's useful to get a better idea of what
repmgrd is actually doing.
Also add documentation section for the log configuration parameters.
GitHub #470, containing change suggested in GitHub #467.
