added NaN test for rising_factorial
added NaN test for rising_factorial
added NaN and derivative doc for rising_factorial in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-rising-factorial
Merge branch 'develop' into feature/test-nan-rising-factorial
Merge pull request #968 from stan-dev/feature/test-nan-rising-factorial

Feature/test nan rising factorial