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.
:ok_hand: I'll restore the trailing slash.  Can't recall why I changed this.