Documentation refinement for Parallel Scans

Hi,

Found documents about parallel scan may be not so accurate.

As said in parallel.smgl:

```
In a parallel sequential scan, the table's blocks will be divided among the cooperating processes. Blocks are handed out one at a time, so that access to the table remains sequential.
```

To my understanding, this was right before. Because we return one block if a worker ask for before commit 56788d2156.
As comments inside table_block_parallelscan_nextpage:
```
Earlier versions of this would allocate the next highest block number to the next worker to call this function.
```
And from commit 56788d2156, each parallel worker will try to get ranges of blocks “chunks".
Access to the table remains sequential inside each worker’s process, but not across all workers or the parallel query.
Shall we update the documents?

Regards,
Zhang Mingli