RE: Mention idle_replication_slot_timeout in pg_replication_slots docs

Dear Fujii-san,

> The pg_replication_slots documentation mentions only max_slot_wal_keep_size
> as a condition under which the wal_status column can show unreserved or lost.
> However, since commit ac0e33136ab, idle_replication_slot_timeout can also
> cause this behavior when it is set. This has not been documented yet.
> https://www.postgresql.org/docs/devel/view-pg-replication-slots.html

Oh, I feel the doc should be also updated.

> So, how about updating the documentation to also mention
> idle_replication_slot_timeout as a factor that can cause wal_status to
> become unreserved or lost? Patch attached.

One comment:

```
<para>
<literal>lost</literal> means that some required WAL files have
been removed and this slot is no longer usable.
</para>
```

IIUC, there is a case that status is "lost" but the required WALs have not been
dropped yet if the slot was invalidated due to the timeout. How about removing the
first part:

```
<literal>lost</literal> means that this slot is no longer usable.
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED