pgsql: Fix nbtree page split rmgr desc routine.

Fix nbtree page split rmgr desc routine.

Include newitemoff in rmgr desc output for nbtree page split records.
In passing, correct an obsolete comment that claimed that newitemoff is
only logged for _L variant nbtree page split WAL records.

Both issues were oversights in commit 2c03216d831, which revamped the
WAL format.

Author: Peter Geoghegan
Backpatch: 9.5-, where the WAL format was revamped.

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9064cc139d10d6e45a370dd8761a02f7e7da36f4

Modified Files
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src/backend/access/rmgrdesc/nbtdesc.c |  4 ++--
src/include/access/nbtree.h           | 12 ++++++------
2 files changed, 8 insertions(+), 8 deletions(-)