Fix confusing NOTICE text in REINDEX CONCURRENTLY

Fix confusing NOTICE text in REINDEX CONCURRENTLY

When performing REINDEX TABLE CONCURRENTLY, if all of the table's indexes
could not be reindexed, a NOTICE message claimed that the table had no
indexes.  This was confusing, so let's change the NOTICE text to something
less confusing.

In passing, also mention in the comment before ReindexRelationConcurrently
that materialized views are supported too and also explain what the return
value of the function means.

Author: Ashwin Agrawal
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CALfoeithHvi13p_VyR8kt9o6Pa7Z=Smi6Nfc2anHnQx5Lj8bTQ@mail.gmail.com