Added information about the cssrewrite filter not working with the @AcmeFooBundle syntax. [kriswallsmith/assetic#53]

https://github.com/kriswallsmith/assetic/issues/53#issuecomment-2976018
[Assetic] Proofreading the Assetic section, rewriting parts, adding more details, evolving it
[#2263] Clarifying when assets are combined thanks to @stof