pgsql: Rework <warning> box about column list combining in logical repl

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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d8209d09170f0029663da4335979b4f49d48b6be

Modified Files
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doc/src/sgml/logical-replication.sgml | 36 +++++++++++++++++++----------------
1 file changed, 20 insertions(+), 16 deletions(-)