Fix docComment asking forever in case of symbol not owned by a class
Remove persistence of PC doc comments.

Comments obtained by parsing are cleared at each typer run.

- fixes memory leak
- makes Scaladoc slower

It's possible (and easy) to reset those comments only at reconciliation,
but the test doesn't trigger reconciliation (it types all the time).
A more composed fix : clearing DocComments at each reconciliation

and Memory leaks test triggers reconciliation
Merge pull request #810 from huitseeker/hotfix/scaladoc-memoryleak

Control persistence of PC doc comments.