pgsql: Doc: Correct description of amcheck example query.

Doc: Correct description of amcheck example query.

The amcheck documentation incorrectly claimed that its example query
verifies every catalog index in the database.  In fact, the query only
verifies the 10 largest indexes (as determined by pg_class.relpages).
Adjust the description accordingly.

Backpatch: 10-, where contrib/amcheck was introduced.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f85537a88dda4aa015f9a1b4a43189395d36b328

Modified Files
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doc/src/sgml/amcheck.sgml | 18 ++++++++----------
1 file changed, 8 insertions(+), 10 deletions(-)