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docs: Clarify how ALTER TABLE .. SET COMPRESSION works.

Justin Pryzby, per a complaint from Michael Paquier. Reviewed byDilip Kumar and by me.Discussion:http://postgr.es/m/20210429040132.GF27406@telsasoft.com

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Original file line number	Diff line number	Diff line change
`@@ -391,7 +391,21 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM`
`391`	`391`	`</term>`
`392`	`392`	`<listitem>`
`393`	`393`	`<para>`
`394`		`- This sets the compression method for a column. The supported compression`
	`394`	`+ This sets the compression method to be used for data inserted into a column.`
	`395`	`+`
	`396`	`+ This does not cause the table to be rewritten, so existing data may still`
	`397`	`+ be compressed with other compression methods. If the table is rewritten with`
	`398`	`+ <command>VACUUM FULL</command> or <command>CLUSTER</command>, or restored`
	`399`	`+ with <application>pg_restore</application>, then all tuples are rewritten`
	`400`	`+ with the configured compression methods.`
	`401`	`+`
	`402`	`+ Also, note that when data is inserted from another relation (for example,`
	`403`	`+ by <command>INSERT ... SELECT</command>), tuples from the source data are`
	`404`	`+ not necessarily detoasted, and any previously compressed data is retained`
	`405`	`+ with its existing compression method, rather than recompressing with the`
	`406`	`+ compression methods of the target columns.`
	`407`	`+`
	`408`	`+ The supported compression`
`395`	`409`	`methods are <literal>pglz</literal> and <literal>lz4</literal>.`
`396`	`410`	`<literal>lz4</literal> is available only if <literal>--with-lz4</literal>`
`397`	`411`	`was used when building <productname>PostgreSQL</productname>.`

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