Expose CodegenWriter's doc writer

When the doc writer is just an anonymous lambda function, it makes sense
to hide it away in the parent implementation. But when it's a class that
satisifies the interface, there may be additional things on it that the
subclasses or callers will want to use. For example, a doc formatter.

Author: JordonPhillips

smithy-codegen-core/src/main/java/software/amazon/smithy/codegen/core/writer/CodegenWriter.java

@@ -85,6 +85,15 @@ public CodegenWriter(DocumentationWriter<T> documentationWriter, U importContain
         this.importContainer = importContainer;
     }
 
+    /**
+     * Gets the documentation writer.
+     *
+     * @return Returns the documentation writer.
+     */
+    public final DocumentationWriter<T> getDocumentationWriter() {
+        return documentationWriter;
+    }
+
     /**
      * Gets the import container associated with the writer.
      *