Update nbtree LP_DEAD item deletion comments.

Update nbtree LP_DEAD item deletion comments.

Comments about the consequences of clearing the BTP_HAS_GARBAGE page
flag bit that apply only to VACUUM were added to code that deals with
opportunistic deletion of LP_DEAD items by commit a760893d.  The same
comment block was added to both _bt_delitems_vacuum() and
_bt_delitems_delete().  Correct _bt_delitems_delete()'s copy of the
comment block.

_bt_delitems_delete() reliably deletes items that were found by caller
to have their LP_DEAD bit set.  There is no question about whether or
not unsetting the BTP_HAS_GARBAGE bit can miss some LP_DEAD items that
were set recently.

Also tweak a related section of the nbtree README.