Feature/test nan log diff exp

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