Message style improvements
* Avoid pointlessly highlighting that an index vacuum was executed by a
parallel worker; user doesn't care.
* Don't give the impression that a non-concurrent reindex of an invalid
index on a TOAST table would work, because it wouldn't.
* Add a "translator:" comment for a mysterious message.
Discussion: https://postgr.es/m/
20201107034943.GA16596@alvherre.pgsql
Reviewed-by: Michael Paquier <michael@paquier.xyz>