pgsql: Update btree_xlog_delete() comments.

Update btree_xlog_delete() comments.

Commit fe97c61c updated LP_DEAD item deletion comments, but missed a
minor discrepancy on the REDO side.  Fix it now.

In passing, don't talk about the btree_xlog_vacuum() behavior within
btree_xlog_delete().  The reliance on XLOG_HEAP2_CLEANUP_INFO records
for recovery conflicts is already discussed within btvacuumpage() and
mentioned again in passing above btree_xlog_vacuum(), which seems
sufficient.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c5f3b53b0ef2e8ae78e7488148c12bfe5939ca17

Modified Files
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src/backend/access/nbtree/nbtree.c  |  7 ++++---
src/backend/access/nbtree/nbtxlog.c | 14 ++------------
2 files changed, 6 insertions(+), 15 deletions(-)