pgsql: 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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fe97c61c8777858cc1a271e657a7d812e100ef00

Modified Files
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src/backend/access/nbtree/README    |  8 ++++----
src/backend/access/nbtree/nbtpage.c | 11 ++---------
2 files changed, 6 insertions(+), 13 deletions(-)