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