Correct gaps in PR #934

- correct XML comment typo introduced in `HtmlHelper` in 56d66c090eaedc33e4838437e7611ba2b9ca8929 (bad merge)
- fix missed `null` requirement for `@Html.RadioButtonFor()` and remove buried `null` check
- add back `Environment.Newline` to `@Html.TextArea()` (was dropped though comment wasn't)
There's a [SO link](http://stackoverflow.com/a/7024077/235591) that says you could use urls in cref. In which case, does it make sense to say 

> The composite format specifier as detailed at <see cref="http://msdn.microsoft.com/en-us/library/txafckwd.aspx" />

or something like that?