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 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
- add `<remarks/>` describing behaviour of input helpers e.g. `CheckBox()`
- add `<remarks/>` explaining "renders"
- add `<remarks/>` containing example expression names

nits:
- "The expression" -> "An expression"
- make examples for `ObjectToDictionary()` and `AnonymousObjectToHtmlAttributes()` more consistent
- move `<typeparam/>` elements (that existed) after all `<param/>` elements
- explain "checked" attributes better, removing some duplicated words from the `RadioButton[For]()` descriptions
- correct `routeValues` description in `GenerateForm()` comments
ViewEngine shouldn't throw when it doesn't know about the extension
Fixes #984
Correct build break
- incorrect XML comment syntax for a few parameter references
[Issue #730] Attribute Routing: Flesh out attributes - Part 1

1. Added a new AttributeRouteInfo class to store all the information for
   actions that are attribute routed.

2. Added a new ReflectedAttributeRouteModel class to store all the information
   related to attribute routes in the ReflectedApplicationModel.

3. Refactored ReflectedControllerModel and ReflectedActionModel to use ReflectedAttributeRouteModel
   instead of just the attribute route template.

4. Refactored ReflectedActionDescriptorProvider to use AttributeRouteInfo and ReflectedAttributeRouteModel
   instead of just the route template.

5. Added a CombineReflectedAttributeRouteModel method in ReflectedAttributeRouteModel that handles
   combining two ReflectedAttributeRouteModel instances.

6. Removed the AttributeRouteTemplate class and moved the methods for combining attribute routes to the
   ReflectedAttributeRouteModel class.

7. Added unit tests for ReflectedActionModel and  ReflectedControllerModel that
   cover the usage of AttributeRouteInfo.

8. Added unit tests for CombineReflectedAttributeRouteModel.
[Issue #730] Attribute Routing: Flesh out attributes - Part 2

1. Unsealed the Http*Attributes so that they can be extended and customized.
2. Added the same constructors as HttpGet to the rest of the Http*Attributes.
3. Added unit tests to validate the implementations for the IActionHttpMethodProvider.
4. Added functional tests to cover extra attribute routing scenarios like a test for an
   action with an HttpDeleteAttribute on it and action with AcceptVerbsAttribute and an
   action with a custom HttpMergeAttribute implemented.