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.