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