doc: Adjust documentation for vacuumdb --missing-stats-only.

The sentence in question gave readers the impression that vacuumdb
removes statistics for a period of time while analyzing, but it's
actually meant to convey that --analyze-in-stages temporarily
replaces existing statistics with ones generated with lower
statistics targets.

Reported-by: Frédéric Yhuel <frederic.yhuel@dalibo.com>
Reviewed-by: Frédéric Yhuel <frederic.yhuel@dalibo.com>
Reviewed-by: "David G. Johnston" <david.g.johnston@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/4b94ca16-7a6d-4581-b2aa-4ea79dbc082a%40dalibo.com
Backpatch-through: 18

doc/src/sgml/ref/vacuumdb.sgml

@@ -282,9 +282,11 @@ PostgreSQL documentation
       <listitem>
        <para>
         Only analyze relations that are missing statistics for a column, index
-        expression, or extended statistics object.  This option prevents
-        <application>vacuumdb</application> from deleting existing statistics
-        so that the query optimizer's choices do not become transiently worse.
+        expression, or extended statistics object.  When used with
+        <option>--analyze-in-stages</option>, this option prevents
+        <application>vacuumdb</application> from temporarily replacing existing
+        statistics with ones generated with lower statistics targets, thus
+        avoiding transiently worse query optimizer choices.
        </para>
        <para>
         This option can only be used in conjunction with