Remove HTML helpers with `IDictionary<string, object>` parameters
- see line 2 of #874
- focus on `TextBox[For]()` and `ValidationSummary()`

related fixes included here:
- `TextArea[For]()` documentation incorrectly indicated their `htmlAttributes` parameters were dictionaries
- handle `htmlAttributes` parameters more consistently; create a dictionary only when necessary
Fix for issue #349
Updating tests using Moq to workaround Roslyn changes

https://roslyn.codeplex.com/workitem/246 affects usage of code with the
latest build of Roslyn with Moq v4.2. The workaround involves ensuring a
closure is created. Updating affected tests to make ToString() calls on
local variables to create these closures.
Remove unused `using`s from 3db0a803062df19e83bdd62b63eba47c016af384 change
- see 3db0a803062df19e83bdd62b63eba47c016af384
Correct XML comment syntax
- fix problems at least with Roslyn compiler and VS IntelliSense or the Object Browser
 - `<see langref="keyword"/>` generates nothing
 - `<example>` at top level (outside `<summary>`) generates nothing
 - curly braces don't become angle brackets outside `<see cref="reference"/>` references
  - yeah, a point @yishaigalatzer asked about in a previous PR
- `<see href="reference"/>` is not valid
- correct some invalid use of angle brackets and remove useless empty elements
- add usings to help comment references resolve (unfortunately VS seems to get confused about these)

Few wording changes beyond
- `"opening </form> tag"` -> `"<form> start tag"`
- `"closing </form> tag"` -> `"</form> end tag"`
Address review comments
- remove and sort usings then qualify any broken XML comment references
- fill in a couple of previously-empty comment elements

Also correct two typos in `HtmlHelper`
Found and fixed additional XML comment issues.  Symptoms included
- generated XML comments such as `<!-- Badly formed XML comment ... -->`, usually indicating an unclosed element
- generated XML attributes such as `cref="!:..."`, indicating a broken reference
 - in a couple of cases we had `<typeparamref cref="TOption"/>`; attribute should be `"name"`

Will create a unit test to ensure XML syntax doesn't degrade going forward.  Separate PR.
- for now, check using `dir -r *.xml | sls '!'`