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Doc: mention that extended stats aren't used for joins

Statistics defined by the CREATE STATISTICS command are only used toassist with the selectivity estimations of base relations, never forjoins. Here we mention this fact in the notes section of the CREATESTATISTICS command.Discussion:https://postgr.es/m/CAApHDvrMuVgDOrmg_EtFDZ=AOovq6EsJNnHH1ddyZ8EqL4yzMw@mail.gmail.comBackpatch-through: 11

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Original file line number	Diff line number	Diff line change
`@@ -167,6 +167,12 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_na`
`167`	`167`	`maintenance. Expression statistics are built automatically for each`
`168`	`168`	`expression in the statistics object definition.`
`169`	`169`	`</para>`
	`170`	`+`
	`171`	`+ <para>`
	`172`	`+ Extended statistics are not currently used by the planner for selectivity`
	`173`	`+ estimations made for table joins. This limitation will likely be removed`
	`174`	`+ in a future version of <productname>PostgreSQL</productname>.`
	`175`	`+ </para>`
`170`	`176`	`</refsect1>`
`171`	`177`
`172`	`178`	`<refsect1 id="sql-createstatistics-examples">`

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