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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ee924698d566223e927cf9d505c1ccdacd7061c8

Modified Files
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doc/src/sgml/ref/vacuumdb.sgml | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)