Disallow single-line `IF expr ELSE` without `THEN`

* Supplement missing block before `ELSE` token only for multi-line `if`.
* Don't prematurely remove `TERMINATOR` before `ELSE` (so we can  differentiate single- and multi-line forms).
* Cleanup: Remove `WHEN` from `EXPRESSION_CLOSE`. (Only `LEADING_WHEN` tokens are preceded by a `TERMINATOR`.)
* Cleanup: Remove really old, inapplicable text from comment.