Cleared up comments in fold functions.

Had a few errors, like the examples in foldr using foldl.
Changed variable names in foldl.m. Cosmetic only.

The changes are to better match the fold documentation for Haskell.
Fixed foldr f, which now has def \x acc.

Prior, the definition of f was \acc x. However, this was counter to the
Haskell definition (and slightly counter-intuitive to a right fold), and
hence the older was flipped.

There are variable name changes as well to aid understanding.
concatMap uses foldr instead of foldl.

Matches Haskell convention, would be robust to infinite lists if we had
them in Matlab....