md: remove doubled description for sync_max, merging it within sync_min/sync_max

After last md.txt edits for sync_min/max, sync_max description became
doubled. Removing 1st copy, merging details into common
sync_min/sync_max section.

Signed-off-by: Roman Ovchinnikov <coolthecold@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>