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.