nbtree: Rename BT_RESERVED_OFFSET_MASK.

nbtree: Rename BT_RESERVED_OFFSET_MASK.

The mask was added by commit 8224de4f, which introduced INCLUDE nbtree
indexes.  The status bits really were reserved initially.  We now use 2
out of 4 of the bits for additional tuple metadata, though.  Rename the
mask to BT_STATUS_OFFSET_MASK.

Also consolidate related nbtree.h code comments about the format of
pivot tuples and posting list tuples.