Manual update for --continue option

Manual update for --continue option

Patch adds to mdadm man the following information:

 --continue
   This option is complementary pair to assembly --freeze-reshape option.
   It is needed when --grow operation is interrupted and it is not restarted
   automatically due to --freeze-reshape usage during array assembly.
   Option --continue has to be used together with -G , ( --grow ) command
   and device that it should be executed on.  All parameters required for
   reshape continuation will be read from array metadata.  If initial
   --grow command had required --backup-file= option to be set,
   continuation option will require to  have  exactly the same backup
   file pointed to also.

   Any other parameter passed together with --continue option will be ignored.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>