chore: updates container-definition user jsdoc (#26231)

Update JSDOC for `user` to clarify that this is a `user string` and not restricted to a user name.

Reason for this change: When viewing the documentation from an IDE it is unclear that something other than a user name can be used for example 100:101 ie UID and GID.

I have not fully read the following:

[CONTRIBUTING GUIDE]: https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md
[DESIGN GUIDELINES]: https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md

This commit/pull request only changes JSDoc.

Closes #26230

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-ecs/lib/container-definition.ts

@@ -294,8 +294,9 @@ export interface ContainerDefinitionOptions {
   readonly readonlyRootFilesystem?: boolean;
 
   /**
-   * The user name to use inside the container.
+   * The user to use inside the container. This parameter maps to User in the Create a container section of the Docker Remote API and the --user option to docker run.
    *
+   * @see https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html#ContainerDefinition-user
    * @default root
    */
   readonly user?: string;