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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master

Details
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https://git.postgresql.org/pg/commitdiff/c6f21b2ac215ad870a95159ea22024f734429622

Modified Files
--------------
doc/src/sgml/logical-replication.sgml | 36 +++++++++++++++++++----------------
doc/src/sgml/mvcc.sgml | 8 +++++---
doc/src/sgml/ref/pg_rewind.sgml | 1 +
3 files changed, 26 insertions(+), 19 deletions(-)