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>` or `<remarks>`) 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"`

See #928 for one thing _not_ fixed in this PR.
Reworded this description very slightly e.g. made the method the subject and added changed "This allows to" to "This allows callers to".  Also removed lots of unnecessary hyphens.