Feature/test nan acos

#### Summary:
Added NaN tests for acos in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. I also added more boundary tests to verify derivative behavior outside of [-1,1].

#### Intended Effect:
Added NaN tests for acos in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/acos, test/unit-agrad-rev/functions/acos

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of acos at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.