pgsql: Doc: Mention about caveats of --concurrently on reindexdb page

Doc: Mention about caveats of --concurrently on reindexdb page

The documentation of REINDEX includes a complete description of
CONCURRENTLY and its advantages as well as its disadvantages, but
reindexdb was not really clear about all that.

From discussion with Tom Lane, based on a report from Andrey Klychkov.

Discussion: https://postgr.es/m/1590486572.205117372@f500.i.mail.ru
Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/38be24228dcdde935e6090ac4d7f85ceea58d914

Modified Files
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doc/src/sgml/ref/reindexdb.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)