SI-6626 make @throws tags create links to exceptions

In scaladoc, this turns exceptions in @throws tags into links
(when it can find the target exception), instead of just showing
the name.
cleanup @throws tags in library and reflect

 - there is no need for explicit links with [[ and ]]
 - there is no need for explicit backquoting
Merge pull request #4089 from gourlaysama/wip/t6626-scaladoc-throws-links

SI-6626 make @throws tags create links to exceptions