Fix Closure comments using ! operator

[jasonsanjose]

Across the Brackets code base we use !function(...) incorrectly in our Closure @param annotations. function types are non-nullable by default. We should remove ! when appropriate and instead use ?function(...) for nullable parameters or function(...)= for optional parameters.

See #2063 (comment).

[pthiess]

Low priority, assigned to @jasonsanjose