Rework <warning> box about column list combining in logical replication
After some copy-edit I made in commit
3a06a79cd137, we have a <sect2>
that only contains a warning box. This doesn't look good. Rework by
moving the sect2 title to be the warning's title, and put the 'id' to it
as well, so that the external reference continues to work.
Backpatch to 15.
In branch master, I also take the opportunity to add titles to a couple
of other warning boxes elsewhere in the documentation.
Discussion: https://postgr.es/m/
20221219164713.ccnlvtkyj6lmshqq@alvherre.pgsql