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.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