MultiSelect "too many options" suggestion incorrect HTML

When implementing JRA-23051, I incorrectly placed the warning that there are too many options to be displayed within the results list element without wrapping it in an li element. Whilst this doesn't negatively affect any of our supported browsers, it is syntactically incorrect.

The best solution would be to wrap the notice element in an li in it's render function. However care must be taken to ensure this element is not clickable (as it will become when wrapped in an li).

Another option is to append it after the results list, however due to the way the code works, this element will not be returned as the function only returns the list and it's contents. Any elements appended to it are ignored.