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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
`@@ -732,8 +732,8 @@ CREATE INDEX people_names ON people ((first_name \|\| ' ' \|\| last_name));`
`732`	`732`
`733`	`733`	`<para>`
`734`	`734`	`Index expressions are relatively expensive to maintain, because the`
`735`		`- derived expression(s) must be computed for each rowuponinsertion`
`736`		`- andwhenever it is updated. However, the index expressions are`
	`735`	`+ derived expression(s) must be computed for each row insertion`
	`736`	`+ andnon-HOT update. However, the index expressions are`
`737`	`737`	`<emphasis>not</emphasis> recomputed during an indexed search, since they are`
`738`	`738`	`already stored in the index. In both examples above, the system`
`739`	`739`	`sees the query as just <literal>WHERE indexedcolumn = 'constant'</literal>`

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