PG13: message style changes

I propose to apply the following changes to messages in pg13.

In 0001, I propose changing messages that were introduced as different 
for parallel vacuum workers.  Frankly I don't understand why we are
bragging about the vacuum being done in a parallel worker; does the user
care?  It seems to me that users are just satisfied to know that the
indexes were scanned; the fact that this was done in a parallel worker
is not of much interest, so why call attention to that?  Therefore, we
can reduce the message to what's emitted in the normal case.

In 0002, I propose to remove the word "concurrently" in an error
message when an invalid index cannot be reindexed.  In fact, the problem
is generic: we just cannot reindex the index at all, regardless of
concurrently or not.  So we can reduce this message to be identical to
the one we throw in the non-concurrent case.

(Dropped 0003 while composing this email.)

Patch 0004 just adds a comment to clarify a message that I found
confusing when doing the translation.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W