Update FAQ with information on uninstall issues

```
<David> Hey guys, having a little problem
<David> brew cask list shows a package, and the directory and symlink are there, but when I try doing brew cask uninstall, I get an error saying the package isn't installed
<David> it happens for some packages but not all
<rolandwalker> David: that is a frequently asked question, and discussed and being worked on in several GitHub issues.
<rolandwalker> The current implementation lacks bookkeeping, and gets confused if a Cask is updated after you have installed it.
<rolandwalker> For the moment, you can use the "--force" option for most of those cases: "brew cask uninstall --force <caskname>"
<David> rolandwalker: thanks! would you mind if I added that to https://github.com/caskroom/homebrew-cask/blob/master/doc/FAQ.md ?
<rolandwalker> Please do, and reference GitHub issues #4688 and #4678.
```