added NaN test for log1m_exp
added NaN test for log1m_exp
added doc for log1m_exp
Merge branch 'develop' into feature/test-nan-log1m-exp
Merge branch 'develop' into feature/test-nan-log1m-exp
Merge pull request #992 from stan-dev/feature/test-nan-log1m-exp

Feature/test nan log1m exp