Addition of a reference about the Apache's ETag bug

Addition of a reference about the [Apache's ETag bug](https://issues.apache.org/bugzilla/show_bug.cgi?id=46538) in the corresponding error text according to a [suggestion of Stéphane Bortzmeyer](https://github.com/mnot/redbot/issues/135).
This is a bit judgemental; REDbot tries not to unnecessarily anger / disparage implementations. Also, Apache's behaviour regarding gzip and ETags changes a bit over different versions, so saying "always" is a bit misleading here. Best to say that "some versions" have this issue.