Various cleanups for @chebfun/diff.m.
Various cleanups for @chebfun/diff.m.

 - Fixed some errors in the documentation.
 - Removed unreachable statement:  dim == 1 or dim == 2, then round(dim)
   == dim, certainly.
 - Added a not-implemented error to the branch governing fractional
   calculus.

asgeirbirkis/chebfun,alshedivat/chebfun,ilovejs/chebfun