Update concrete-syntax.md

Document the workaround for now.

Author: Apanatshka

content/howtos/stratego/concrete-syntax.md

@@ -108,10 +108,13 @@ Note that the constructor `FromTerm` indicates that productions represent anti-q
 Using anti-quotations might make the fragment of the target language quite verbose. Therefore, it is also possible to define variables as shortcuts to anti-quotations. For example, the productions below define variables to reference anti-quotations to `Type` fragments. That is, instead of reference to a Stratego variable `X` by using `~type:X`, one may name this variable `t_1` which corresponds to a variable for a non-terminal `Type`.
 
 ```sdf3
-variables
-  Type = "t_" [0-9\']* {prefer}
+context-free syntax
+  Type.meta-var = "t_" [0-9\']* {prefer}
 ```
 
+!!! note "This special `meta-var` constructor is a workaround for a bug in the `variables` section translation in the SDF3 compiler."
+
+
 The `prefer` annotation indicates that in case of an ambiguity, the variable production should be preferred.
 
 Using the three types of productions above, it is possible to specify which fragments one wants to write using concrete syntax and which symbols may appear inside these fragments as Stratego variables (using anti-quotation or variables with a specific name).