Changes since last update:
- Fix a small math error in the inode btree section.
- Document the new inode btree counter feature.
- Document the new timestamp extension feature.

design: document changes for the bigtime feature

Document the changes to the ondisk format when we enable the bigtime
feature.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>