Feature/test nan falling factorial

#### Summary:
Added NaN tests for falling_factorial in math/functions, agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN.

#### Intended Effect:
Added NaN tests for falling_factorial in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/falling_factorial, test/unit-agrad-rev/functions/falling_factorial

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of falling_factorial at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.