More documentation on code coverage/hpc

Our code coverage documentation is a bit woeful, only mentioning the existence of `--enable-coverage`. Here is the bare minimum:

* The info in http://stackoverflow.com/questions/28416827/cabal-and-hpc-and-errors-when-running-tests-with-code-coverage
* More detail about how coverage integrates with test suites, calling out explicitly that other use cases of coverage are not directly supported (#3902)
* Where the reports/tix files go