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)
Simplify some XML comment references
- follow VS fixup suggestion and change e.g. `<see cref="System.String"/>` -> `<see cref="string"/>`