pgsql: docs: adjust tag indenting and add MERGE mention

docs: adjust tag indenting and add MERGE mention

Discussion: /messages/by-id/CAMpnoC4_WsY3gsY+ud-Z0GDbafR=K7t7cXn2gatEqFnsRNY3yQ@mail.gmail.com

Author: Will Mortensen

Backpatch-through: 15

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/39abe1401e3180a336424c30ece949607322f4a6

Modified Files
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doc/src/sgml/mvcc.sgml | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)