Feature/test nan abs

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