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 923936e ("iomap: Fix conflicting values of iomap flags")

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

Author: riteshharjani

Documentation/filesystems/iomap/design.rst

@@ -243,13 +243,25 @@ The fields are as follows:
      regular file data.
      This is only useful for FIEMAP.
 
-   * **IOMAP_F_PRIVATE**: Starting with this value, the upper bits can
-     be set by the filesystem for its own purposes.
+   * **IOMAP_F_BOUNDARY**: This indicates I/O and its completion must not be
+     merged with any other I/O or completion. Filesystems must use this when
+     submitting I/O to devices that cannot handle I/O crossing certain LBAs
+     (e.g. ZNS devices). This flag applies only to buffered I/O writeback; all
+     other functions ignore it.
+
+   * **IOMAP_F_PRIVATE**: This flag is reserved for filesystem private use.
 
    * **IOMAP_F_ANON_WRITE**: Indicates that (write) I/O does not have a target
      block assigned to it yet and the file system will do that in the bio
      submission handler, splitting the I/O as needed.
 
+   * **IOMAP_F_ATOMIC_BIO**: This indicates write I/O must be submitted with the
+     ``REQ_ATOMIC`` flag set in the bio. Filesystems need to set this flag to
+     inform iomap that the write I/O operation requires torn-write protection
+     based on HW-offload mechanism. They must also ensure that mapping updates
+     upon the completion of the I/O must be performed in a single metadata
+     update.
+
    These flags can be set by iomap itself during file operations.
    The filesystem should supply an ``->iomap_end`` function if it needs
    to observe these flags: