Documentation: iomap: Add missing flags description

Documentation: iomap: Add missing flags description

Let's document the use of these flags in iomap design doc where other
flags are defined too -

- IOMAP_F_BOUNDARY was added by XFS to prevent merging of I/O and I/O
  completions across RTG boundaries.
- IOMAP_F_ATOMIC_BIO was added for supporting atomic I/O operations
  for filesystems to inform the iomap that it needs HW-offload based
  mechanism for torn-write protection.

While we are at it, let's also fix the description of IOMAP_F_PRIVATE
flag after a recent:
commit 923936efeb74b3 ("iomap: Fix conflicting values of iomap flags")

Signed-off-by: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Link: https://lore.kernel.org/8d8534a704c4f162f347a84830710db32a927b2e.1744432270.git.ritesh.list@gmail.com
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>