`@Html.DisplayName()`, `.DisplayText()`, `.Id()`, `.Name()`, `.Value()` return `string`
- fixes #566 and part of #847
- allows compositions such as `@Html.Label("property", Html.Id("property"))`
  and `@Html.Raw(Html.DisplayText("property"))` (this one is not recommended)
- adjust XML comments to match
- add missing XML comments to `HtmlHelperValueExtensions`
- nit: `TInnerModel` -> `TModelItem`

- increase XML comment consistency for changed methods
 - generally make wording more consistent e.g. how we use words such as
   "returns"
 - use `<see langref="string|true|false|null"/>` more
Remove useless `.ToString()` calls in tests
Add `HtmlHelperDisplayTextTest` and `HtmlHelperValueExtensionsTest` classes