Обсуждение: pgsql: docs: adjust multi-column most-common-value statistics

docs:  adjust multi-column most-common-value statistics

This commit adds a mention that the order of columns specified during
multi-column most-common-value statistics is insignificant, and tries to
simplify examples.

Discussion: https://postgr.es/m/20190828162238.GA8360@momjian.us

Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/737f226842748cf946cad3bd30aadfd0c03ad16f

Modified Files
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doc/src/sgml/perform.sgml               | 6 +++---
doc/src/sgml/ref/create_statistics.sgml | 3 ++-
2 files changed, 5 insertions(+), 4 deletions(-)