Thread: pgsql: Link to MVCC docs in MERGE docs

Link to MVCC docs in MERGE docs

In addition, this moves the new paragraph in the MVCC page upwards, for
a more consistent flow; some minor markup mistakes, style issues and
typos are fixed too.

Per comments from Justin Pryzby.

Discussion: https://postgr.es/m/20220511163350.GL19626@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ffffeebf244770a9450d9fe2597abde725a9b984

Modified Files
--------------
doc/src/sgml/mvcc.sgml      | 71 +++++++++++++++++++++++----------------------
doc/src/sgml/ref/merge.sgml |  4 +++
2 files changed, 40 insertions(+), 35 deletions(-)