added NaN test for log1m_inv_logit
added NaN test for log1m_inv_logit
added doc for log1m_inv_logit
Merge branch 'develop' into feature/test-nan-log1m-inv-logit
Merge branch 'develop' into feature/test-nan-log1m-inv-logit
Merge pull request #991 from stan-dev/feature/test-nan-log1m-inv-logit

Feature/test nan log1m inv logit