Pimcore_Image_HtmlToImage bugfix: document previews without xvfb

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3907 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version (2969) ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3908 1f8fe7d8-47f0-464c-8d0a-336f4953ab05