useless sorting constraints

[matsc-at-sics-se]

The following are pretty useless, aren't they?

predicate strictly_decreasing(array [$X] of var bool: x)
predicate strictly_increasing(array [$X] of var bool: x)

[Dekker1]

Yes, it does not often make sense. I'll remove it from the documentation (since the integer version is clearer), the fzn_* definitions, and replace it with a simpler definition that issues a warning to the modeller should you call strictly_increasing on an array or length 3 or higher.