Feature/test nan phi approx

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