btrfs-progs: docs: expand abbreviations for clarity

Replace the use of IOW as abbreviation of "In other words" with the
original expanded meaning, as users who don't have English as their
first language may not know what it means.

Pull-request: #966
Signed-off-by: David Sterba <dsterba@suse.com>

Author: kchr

Documentation/ch-volume-management-intro.rst

@@ -84,9 +84,9 @@ What changed:
 * available data space decreased by 3GiB, usable roughly (50 - 3) + (100 - 3) = 144 GiB
 * metadata redundancy increased
 
-IOW, the unequal device sizes allow for combined space for data yet improved
-redundancy for metadata. If we decide to increase redundancy of data as well,
-we're going to lose 50GiB of the second device for obvious reasons.
+In other words, the unequal device sizes allow for combined space for data yet
+improved redundancy for metadata. If we decide to increase redundancy of data as
+well, we're going to lose 50GiB of the second device for obvious reasons.
 
 .. code-block:: bash