pgsql: Revise MERGE documentation

Revise MERGE documentation

Add a note about the additional privileges required after the fix in
4989ce72644b (wording per Tom Lane); also change marked-up mentions of
"target_table_name" to be simply "the target table" or the like.  Also,
note that "join_condition" is scouted for requisite privileges.

Backpatch to 15.

Discussion: https://postgr.es/m/202402211653.zuh6objy3z72@alvherre.pgsql

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c4bd6ff57c9a7b188cbd93855755f1029d7a5662

Modified Files
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doc/src/sgml/ref/merge.sgml | 50 +++++++++++++++++++++++----------------------
1 file changed, 26 insertions(+), 24 deletions(-)