Feature/test nan phi agrad

#### Summary:
Added NaN tests for Phi in agrad/fwd/functions and agrad/rev/functions doc is in #939.

#### Intended Effect:
Added NaN tests for Phi in agrad/fwd/functions and agrad/rev/functions doc is in #939.

#### How to Verify:
Run test/unit-agrad-fwd/functions/Phi and test/unit-agrad-rev/functions/Phi,

#### Side Effects:
None.

#### Documentation:
See #939.

#### Reviewer Suggestions:
Anyone.