Improve handling of backups in chebfun.update().

 - Add documentation indicating where the backup will be placed and what
   name it will have so that the user can move any files of the same
   name that may exist there out of the way first.  (Unlikely, I know,
   but it wouldn't be polite to overwrite them without there at least
   being some indication somewhere that this could happen.)

 - Prompt the user for a backup in case the --backup option was not
   passed.  This is especially important if someone calls this via
   "chebfun --update', as that syntax currently does not accept the
   --backup flag.

 - Error if the backup location is a subdirectory of the chebfunroot.
   (It'll get deleted otherwise!)