added NaN test for log_inv_logit
added NaN test for log_inv_logit
added doc for log_inv_logit
Merge branch 'develop' into feature/test-nan-log-inv-logit
Merge branch 'develop' into feature/test-nan-log-inv-logit
Merge pull request #988 from stan-dev/feature/test-nan-log-inv-logit

Feature/test nan log inv logit