Improve XML comments in `HtmlHelper` and related classes and interfaces

- #847 line 1: copy XML comments from interfaces to extension methods
- lots of wording changes to be more consistent
 - core text split from PR #866, where @rynowak and I hashed out the words
 - to extent possible, reuse words between method descriptions

- add `<remarks>` describing behaviour of input helpers e.g. `CheckBox()`
- add a few missing `<param/>` and `<typeparam/>` elements and fill in empty ones
- display more HTML elements as tags
- use `<c>true|false|null</c>` more often

nits:
- "The expression" -> "An expression"
- make examples for `ObjectToDictionary()` and `AnonymousObjectToHtmlAttributes()` more consistent
- move `<typeparam/>` elements (that existed) after all `<param/>` elements
Think this is fine as-is because this really is about conversion.  The term is contrasted with "formats" when there's a `format` parameter.  Here the comment mostly means we're passing `null` to the real method and, therefore, formatting doesn't happen.
This wording is also consistent with the input helpers that accept a `format` parameter.  Lots more going on there.