[DOCS] HOT - correct claim about indexes not referencing old line pointers

Hello,

While working on my talk for PGConf.NYC next week I came across this
bullet in the docs on heap only tuples:

> Old versions of updated rows can be completely removed during normal
> operation, including SELECTs, instead of requiring periodic vacuum
> operations. (This is possible because indexes do not reference their page
> item identifiers.)

But when a HOT update happens the entry in an (logically unchanged)
index still points to the original heap tid, and that line item is
updated with a pointer to the new line pointer in the same page.

Assuming I'm understanding this correctly, attached is a patch
correcting the description.

Thanks,
James Coleman