Updated lang_collectors for future language

It's not clear whether anything has actually changed, but I did add a
note that collectors can't be used in arrays (https://github.com/puppetlabs/puppet-specifications/blob/master/language/catalog_expressions.md#relationships).
Updated lang_comments for future language

No changes at all.
Updated lang_conditional for future language

The major change here is that selectors take control expressions rather
than control variables. This was an odd inconsistency between selectors
and case statements in earlier versions.
Updated lang_containment for future language

The only real change I made here was to remove the known
issue (PUP-1597). I couldn't find any indication that the behavior is
different in the future parser/evaluator.