(bug 26716) Provide a link for setup instructions for external editor related prefere...

(bug 26716) Provide a link for setup instructions for external editor related preferences. As it stands
no one really knows what they do. This can cause confusion if they get accidently checked or whatever.

Personally I think we should maybe remove these preferences altogether, since external editor can be used without them,
and its unclear how many people use it, but that requires more research into how popular the pref is, etc.