Feature/test nan fmin

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