btrfs-progs: convert: Add comment for the overall convert design
Convert is now a little complex due to that fact we need to separate
metadata and data chunks for different profiles.
Add a comment with ascii art explaining the whole design and point
out the really complex part, so any newcomers interested in convert can
get a quick overview of it before digging into the hard to read code.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ wording and formatting adjustments ]
Signed-off-by: David Sterba <dsterba@suse.com>