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doc: clarify when expression indexes evaluate their expressions

Only non-HOT updates evaluate the index expression.Reported-by: Chris LowderDiscussion:https://postgr.es/m/163967385701.26064.15365003480975321072@wrigleys.postgresql.orgBackpatch-through: 10

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Original file line number	Diff line number	Diff line change
`@@ -748,8 +748,8 @@ CREATE INDEX people_names ON people ((first_name \|\| ' ' \|\| last_name));`
`748`	`748`
`749`	`749`	`<para>`
`750`	`750`	`Index expressions are relatively expensive to maintain, because the`
`751`		`- derived expression(s) must be computed for each rowuponinsertion`
`752`		`- andwhenever it is updated. However, the index expressions are`
	`751`	`+ derived expression(s) must be computed for each row insertion`
	`752`	`+ andnon-HOT update. However, the index expressions are`
`753`	`753`	`<emphasis>not</emphasis> recomputed during an indexed search, since they are`
`754`	`754`	`already stored in the index. In both examples above, the system`
`755`	`755`	`sees the query as just <literal>WHERE indexedcolumn = 'constant'</literal>`

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