Small fixes for CHECKPOINT FLUSH_UNLOGGED

Hi,

I just tested “Add FLUSH_UNLOGGED option to CHECKPOINT command”, and found no functional issue. I only noticed a couple of small issues.

1. Tab-completion for FLUSH_UNLOGGED misses its boolean value

FLUSH_UNLOGGED can take a boolean value, but tab completion does not suggest ON/OFF for it. For comparison, REPACK does:
```
evantest=# checkpoint (FLUSH_UNLOGGED o

evantest=# repack (concurrently O
OFF ON
```The fix just follows the existing code for REPACK. After the fix:
```
evantest=# checkpoint (flush_unlogged O
OFF ON
```

2. The CHECKPOINT doc still says “All data files will be flushed to disk”

After introducing FLUSH_UNLOGGED, CHECKPOINT by default no longer flushes dirty buffers of unlogged relations, but checkpoint.sgml still says “All data files will be flushed to disk.” That seems stale, so I updated the doc.

These are two small changes, but to make the patch easier to process, I split them into two commits.

Best reagards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/