Doc: add rules for UI text

Use double quotes if you need quotes.
Use QDir::toNativeSeparators() for file and directory names that you
pass to \c{tr().arg().
Do not use markup that spans the whole string.

Change-Id: I364f5b12d2d9c68c104bf8ff6a12d9e4fbab4514
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>