Enhance documentation for censored data modeling

Stan has went a long way since this manual section was originally written, so now censoring could be implemented more efficiently than it's described there.

I've made a few changes:
- it's confusing that right-censored y_obs is declared to be positive, whereas it should be unconstrained
- it should be easier to read when it's explicitly specified that the examples deal with right-censored data; and left-censored case is also discussed
- now array sampling operator could be used instead of loops
- now there is normal_ccdf_log to be used instead of log1m( normal_cdf() )