repack: clarify final phase of concurrent mode in file header comment

Hi,

The file header comment in repack.c briefly explains the workflow of concurrent mode as:

1. Lock the table with ShareUpdateExclusiveLock
2. Do an initial copy as non-concurrent mode
3. While the initial copy is going on, accumulates concurrent changes to a file
4. After the initial copy, read the file and re-apply the changes
5. Upgrade to AccessExclusiveLock and swap the relfilenodes

When I read the comment, I got confused at step 4. While that re-apply pass is running, concurrent changes can still occur, so what happens to those changes?

After reading the code, I see that there is actually an extra step after upgrading to AccessExclusiveLock to process the remaining changes.

I am afraid other readers may have the same confusion, so I am proposing this small patch to make the file header comment a bit clearer.

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