A small correction to doc and comment of FSM for indexes

Hi,

This patch fixes a couple of small inaccuracies in the doc and the comment for FSM about index handling.

1. In the doc for pg_freespacemap, it currently says:

> For indexes, what is tracked is entirely-unused pages, rather than free space within pages. Therefore, the values are not meaningful, just whether a page is full or empty.

However, as what is tracked is entirely-unused pages, the values mean whether a page is "in-use or empty", rather than "full or empty".

2. In indexfsm.c the header comment says:

> * This is similar to the FSM used for heap, in freespace.c, but instead
> * of tracking the amount of free space on pages, we only track whether
> * pages are completely free or in-use. We use the same FSM implementation
> * as for heaps, using BLCKSZ - 1 to denote used pages, and 0 for unused.

However, in the code we see that used pages are marked with 0:

> /*
> * RecordUsedIndexPage - mark a page as used in the FSM
> */
> void
> RecordUsedIndexPage(Relation rel, BlockNumber usedBlock)
> {
> RecordPageWithFreeSpace(rel, usedBlock, 0);
> }

And free pages are marked with BLCKSZ - 1:

> /*
> * RecordFreeIndexPage - mark a page as free in the FSM
> */
> void
> RecordFreeIndexPage(Relation rel, BlockNumber freeBlock)
> {
> RecordPageWithFreeSpace(rel, freeBlock, BLCKSZ - 1);
> }

And so, this patch also fixes the comment's "using BLCKSZ - 1 to denote used pages, and 0 for unused" to be "using 0 to denote used pages, and BLCKSZ - 1 for unused".

While these changes are minor, I've seen how this can cause a bit of confusion, and it would be good to clarify it.

Best regards,

Alex Friedman