chore(rds): document DatabaseInstance.fromDatabaseInstanceAttributes …

…method

Ensure `DatabaseInstance.fromDatabaseInstanceAttributes` is included in docs by moving it from the abstract `DatabaseInstanceBase` to the concrete `DatabaseInstance` class.

Thx @skinny85 for the suggestion in aws#11817.

Fixes aws#11817

packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -111,26 +111,6 @@ export interface DatabaseInstanceAttributes {
  * A new or imported database instance.
  */
 export abstract class DatabaseInstanceBase extends Resource implements IDatabaseInstance {
-  /**
-   * Import an existing database instance.
-   */
-  public static fromDatabaseInstanceAttributes(scope: Construct, id: string, attrs: DatabaseInstanceAttributes): IDatabaseInstance {
-    class Import extends DatabaseInstanceBase implements IDatabaseInstance {
-      public readonly defaultPort = ec2.Port.tcp(attrs.port);
-      public readonly connections = new ec2.Connections({
-        securityGroups: attrs.securityGroups,
-        defaultPort: this.defaultPort,
-      });
-      public readonly instanceIdentifier = attrs.instanceIdentifier;
-      public readonly dbInstanceEndpointAddress = attrs.instanceEndpointAddress;
-      public readonly dbInstanceEndpointPort = attrs.port.toString();
-      public readonly instanceEndpoint = new Endpoint(attrs.instanceEndpointAddress, attrs.port);
-      public readonly engine = attrs.engine;
-      protected enableIamAuthentication = true;
-    }
-
-    return new Import(scope, id);
-  }
 
   public abstract readonly instanceIdentifier: string;
   public abstract readonly dbInstanceEndpointAddress: string;
@@ -975,6 +955,28 @@ export class DatabaseInstance extends DatabaseInstanceSource implements IDatabas
 
     this.setLogRetention();
   }
+
+  /**
+   * Import an existing database instance.
+   */
+  public static fromDatabaseInstanceAttributes(scope: Construct, id: string, attrs: DatabaseInstanceAttributes): IDatabaseInstance {
+    class Import extends DatabaseInstanceBase implements IDatabaseInstance {
+      public readonly defaultPort = ec2.Port.tcp(attrs.port);
+      public readonly connections = new ec2.Connections({
+        securityGroups: attrs.securityGroups,
+        defaultPort: this.defaultPort,
+      });
+      public readonly instanceIdentifier = attrs.instanceIdentifier;
+      public readonly dbInstanceEndpointAddress = attrs.instanceEndpointAddress;
+      public readonly dbInstanceEndpointPort = attrs.port.toString();
+      public readonly instanceEndpoint = new Endpoint(attrs.instanceEndpointAddress, attrs.port);
+      public readonly engine = attrs.engine;
+      protected enableIamAuthentication = true;
+    }
+
+    return new Import(scope, id);
+  }
+
 }
 
 /**