more descriptive message for process termination due to max_slot_wal_keep_size

Hello.

As complained in pgsql-bugs [1], when a process is terminated due to
max_slot_wal_keep_size, the related messages don't mention the root
cause for *the termination*. Note that the third message does not
show for temporary replication slots.

[pid=a] LOG: terminating process x to release replication slot "s"
[pid=x] LOG: FATAL: terminating connection due to administrator command
[pid=a] LOG: invalidting slot "s" because its restart_lsn X/X exceeds max_slot_wal_keep_size

The attached patch attaches a DETAIL line to the first message.

> [17605] LOG: terminating process 17614 to release replication slot "s1"
+ [17605] DETAIL: The slot's restart_lsn 0/2C0000A0 exceeds max_slot_wal_keep_size.
> [17614] FATAL: terminating connection due to administrator command
> [17605] LOG: invalidating slot "s1" because its restart_lsn 0/2C0000A0 exceeds max_slot_wal_keep_size

Somewhat the second and fourth lines look inconsistent each other but
that wouldn't be such a problem. I don't think we want to concatenate
the two lines together as the result is a bit too long.

> LOG: terminating process 17614 to release replication slot "s1" because it's restart_lsn 0/2C0000A0 exceeds max_slot_wal_keep_size.

What do you think about this?

[1] https://www.postgresql.org/message-id/20211214.101137.379073733372253470.horikyota.ntt%40gmail.com

--
Kyotaro Horiguchi
NTT Open Source Software Center