Feature/test nan acosh

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