added NaN test for log_diff_exp
added NaN test for log_diff_exp
added doc for log_diff_exp
Merge branch 'develop' into feature/test-nan-log-diff-exp
Merge branch 'develop' into feature/test-nan-log-diff-exp
Merge pull request #987 from stan-dev/feature/test-nan-log-diff-exp

Feature/test nan log diff exp