[mlir][doc] Add basic doc for extraTraitClassDeclaration.

Fixes llvm#61247.

Author: jpienaar

mlir/docs/Interfaces.md

@@ -409,6 +409,10 @@ comprised of the following components:
         the instance is the interface class. In the trait declaration, the
         type of the instance is the concrete entity class
         (e.g. `IntegerAttr`, `FuncOp`, etc.).
+*   Extra Trait Class Declarations (Optional: `extraTraitClassDeclaration`)
+    -   Additional C++ code that is injected into the interface trait
+        declaration.
+    -   Allows the same replacements as extra shared class declarations.
 
 `OpInterface` classes may additionally contain the following: