VE-675: prevent IE 11 from using CK 3.6.1 (browser is incompatible).
Update comment about compatibility mode

Note that there are separate options in the developer tools that allow
switching the document mode and user agent string separately.
Merge pull request #2341 from Wikia/VE-675

VE-675: prevent IE 11 from using CK 3.6.1 (browser is incompatible).