Extend the UserIdentity article

[ighristov]

Description

The UserIdentity API ref and the related UserIdentity Expression article do not contain information how the expression behaves in non-web environment.

The client in this ticket got confused and expects to have the same output when the report is rendered in both RS and RSA. Our docs do not specifically say what happens in such cases.

Suggestion

• Add article in Report Server docs explaining how UserIdentity is resolved and evaluated in both RS and RSA.
• Extend the UserIdentity description in Reporting docs, cross-linking the RS article.