Added paragraph in FAQ for working around incorrect presentation compiler errors

Added paragraph in FAQ for restarting presentation compiler when incorrect
typechecking errors are reported by the presentation compiler.

See scala-ide/scala-ide@b97beec5cf50b48a4f87d400ddf039028196b759
Merge pull request #71 from scala-ide/scala-ide-pr-468/restart-presentation-compiler-doc

Added paragraph in FAQ for working around incorrect presentation compiler errors